An attack on a U.S. base in Mosul (search) Tuesday killed at least 22 people — among them as many as 19 U.S. soldiers — and wounded more than 60, Pentagon officials said. A radical Islamic group claimed responsibility for the rocket attack in the northern Iraqi city.

It was the deadliest single incident for U.S. troops since the start of the war in Iraq.

Three other soldiers of unknown nationality also died in the attack on the camp, which is based outside the predominantly Sunni Muslim city about 220 miles north of Baghdad, a military spokesman said.

The Ansar al-Sunnah Army (search) claimed responsibility for the attack in a statement published on the Internet. The statement said the attack was a "martyrdom operation" targeting a mess hall.

Ansar al-Sunnah is believed to be a fundamentalist group whose goal is to turn Iraq into a tightly controlled Islamic state like Afghanistan's former Taliban (search) regime. In August, the Sunni Muslim group claimed responsibility for the beheading of 12 Nepalese hostages.

The dead included U.S. military personnel, U.S. contractors, foreign national contractors and Iraqi army members, said Brig. Gen. Carter Ham, commander of Task Force Olympia in Mosul.

Halliburton spokeswoman Wendy Hall said seven of the dead were from its KBR subsidiary or its subcontractors. Halliburton and its subcontractors have lost 62 personnel while performing services under their contracts in the Kuwait-Iraq region, she said.

The base, also known as the al-Ghizlani military camp (search), is used by both U.S. troops and the interim Iraqi government's security forces. Lt. Bill Costello, speaking from Fort Lewis, Wash., where many of the soldiers were based, said the troops stationed at what the U.S. military calls Forward Operating Base Marez (search) were tasked with providing stability and support in the northern region of Iraq.

"The [U.S.] soldiers and their Iraqi counterparts took action to evacuate those who were wounded and prevent a further loss of life," Costello said. The soldiers stationed at the base had recently been deployed to replace troops from the 3rd Brigade, 2nd Infantry Division, many of whom had participated in the U.S.-led invasion of Iraq more than a year ago, Costello said.

The cause of the blast, which happened at 12 noon local time, was under investigation.

Pentagon officials told FOX News' Bret Baier that they believed three separate rockets caused the explosions.

The attack came after a surprise visit to Baghdad by British Prime Minister Tony Blair (search), who met with his Iraqi counterpart Ayad Allawi (search). Also Tuesday, kidnappers released two French reporters who had been held hostage in Iraq for four months.

President Bush, speaking outside Walter Reed Army Medical Center, said he and first lady Laura Bush offered their condolences. Bush also said the military mission underway in Iraq was a "vital mission for peace."

"I'm confident democracy will prevail in Iraq. I know a free Iraq will lead to a more peaceful world," Bush said.

Embedded Reporter Describes Grisly Scene

Jeremy Redmon, a reporter for the Richmond, Va., Times-Dispatch embedded with the troops in Mosul, reported that 24 were killed in the attack, including two soldiers from the Richmond-based 276th Engineer Battalion, which had just sat down to eat.

He reported 64 were wounded, and civilians may have been among them, he said.

The attack occurred at lunchtime in a large mess tent crowded with soldiers, according to Redmon. He said the force of the explosions knocked soldiers off their feet and out of their seats.

Amid the screaming and thick smoke in the tent, soldiers turned their tables upside down, placed the wounded on them and gently carried them into the parking lot, Redmon said.

Scores of troops crammed into concrete bomb shelters, while others wandered around in a daze and collapsed, he said.

"I can't hear! I can't hear!" one female soldier cried as a friend hugged her.

The shelling blew a huge hole in the roof of the tent, and puddles of blood, lunch trays and overturned tables and chairs covered the floor, Redmond reported.

Near the front entrance, troops tended a soldier with a serious head wound, but within minutes, they zipped him into a black body bag, he said. Three more bodies were in the parking lot.

"It is indeed a very, very sad day," Ham said.

It made no difference whether the casualties were soldiers or civilians, Americans or Iraqis, Ham said. "They were all brothers in arms taking care of one another," he said.

White House spokesman Scott McClellan, responding to a question as to how Iraqis will be able to go to some 9,000 polling places on Jan. 30 if U.S. troops can't secure their own bases from attacks, said there was "security and peace" in 15 of 18 provinces in Iraq.

Mosul a Staging Ground for Insurgents

Mosul, Iraq's third-largest city, was relatively peaceful in the immediate aftermath of the fall of Saddam Hussein's regime last year. But insurgent attacks have increased dramatically in the past several months and particularly since the U.S.-led military operation in November to retake the restive city of Fallujah from guerrillas.

Mosul was the scene of the deadliest single incident for U.S. troops in Iraq. On Nov. 15, 2003, two Black Hawk helicopters (search) collided over the city, killing 17 soldiers and injuring five. The crash occurred as the two choppers maneuvered to avoid ground fire from insurgents.

Earlier in the day, hundreds of students demonstrated in the center of the city, demanding that U.S. troops cease breaking into homes and mosques in the city.

Also Tuesday, Iraqi security forces repelled another attack by insurgents as they attempted to seize a police station in the center of the city, the U.S. military said in a statement.

"An Iraqi police station came under attack by indirect and small arms fire during a coordinated effort by insurgent fighters to overrun the station in central Mosul," the statement said. "The Iraqi police successfully repelled the attack."

Blair Pays Surprise Visit to Baghdad

In Baghdad Tuesday, British Prime Minister Tony Blair urged Iraqis to support national elections and described violence here as a "battle between democracy and terror" during a surprise visit to the Iraqi capital.

Blair held talks with Interim Prime Minister Ayad Allawi and Iraqi election officials, who he called heroes for carrying out their work despite attacks by insurgents. Three members of Iraq's election commission were dragged from the car and killed this week in Baghdad.

"I said to them that I thought they were the heroes of the new Iraq that's being created, because here are people who are risking their lives every day to make sure that the people of Iraq get a chance to decide their own destiny," Blair said during a joint news conference with Allawi.

Blair, who has paid a political price for going to war in Iraq, defended the role of Britain's 8,000 troops by referring to terrorism.

"If we defeat it here, we deal it a blow worldwide," he said. "If Iraq is a stable and democratic country, that is good for the Middle East, and what is good for the Middle East, is actually good for the world, including Britain.

Blair, whose trip to Iraq hadn't been disclosed for security reasons, urged Iraqis to back the Jan. 30 national vote.

"Whatever people's feelings and beliefs about the removal of Saddam Hussein, and the wisdom of that, there surely is only one side to be on in what is now very clearly a battle between democracy and terror," he said.

The British leader said that apart from the insurgents' violence, "there is another choice for Iraq: the choice is democracy, the choice is freedom, and our job is to help them get there because that's what they want."

Allawi said his government was committed to holding the elections as scheduled next month, despite calls for their postponement owing to the violence.

"We have always expected that the violence would increase as we approach the elections," Allawi said. "We now are on the verge, for the first time in history, of having democracy in action in this country."

Blair said that as the U.S.-led multinational force, in which British troops are serving, trains and improves the Iraqi security forces, "that brings forward the day that the multinational force can leave" Iraq. The presence of foreign troops in Iraq is strongly opposed across the Arab world.

Blair flew into the Iraqi capital about 11 a.m. aboard a British military transport aircraft from Jordan. A Royal Air Force Puma helicopter flew from Baghdad airport to the city center, escorted by U.S. Black Hawk helicopters.

It was Blair's first visit to Baghdad and his third to Iraq since the dictator Saddam Hussein was toppled in April 2003. Blair visited British troops stationed around the southern Iraqi city of Basra in mid-2003 and in January. President Bush had paid a late night visit to U.S. troops in Baghdad in November 2003.

The British leader was a key supporter of the U.S.-led invasion of Iraq that toppled Saddam. His decision to back the U.S. offensive angered many lawmakers in his governing Labour Party and a large portion of the British public.

Britain has some 9,800 troops in Iraq, stationed mostly around Basra. It is the second largest contributor to the multinational force after the United States.

Before meeting Allawi, Blair met the commander of the multinational force, U.S. Army Gen. George W. Casey, and the senior British military officer in Iraq, Lt. Gen. John Kiszely.

In ongoing violence on Tuesday, a U.S. jet bombed a suspected insurgent target in central Iraq and gunmen assassinated an Iraqi nuclear scientist north of Baghdad.

In Baghdad, two French reporters held hostage for four months in Iraq were released and handed over to French authorities, the Foreign Ministry said.

Elsewhere, five American soldiers and an Iraqi civilian were wounded when the Humvee they were traveling in was hit by a car bomb near Hawija, 150 miles north of Baghdad, the U.S. military said.

The bloodshed came a day after Allawi blamed the upsurge of violence on a campaign by insurgents to foment sectarian civil war as well as derail legislative elections set for Jan. 30.

Allawi said the mainly Sunni Muslim insurgents, blamed for Sunday's bombings in the Shiite holy cities of Najaf and Karbala, want to "create ethnic and religious tensions, problems and conflicts ... to destroy the unity of this country."

The coordinated bombings killed 67 people and injured almost 200 in one of the bloodiest attacks on civilians this year.

In other violence Tuesday, a U.S. jet bombed a suspected insurgent target west of Baghdad. Hamdi Al-Alosi, a doctor in a hospital in the city of Hit, said four people were killed and seven injured in the strike. He said the attack damaged several cars and two buildings. A U.S. military spokesman could not confirm the casualties.

Elsewhere, five American soldiers and an Iraqi civilian were wounded when the Humvee they were traveling in was hit by a car bomb near Hawija, 150 miles north of Baghdad, the U.S. military said.

In Baqouba, a city 35 miles northeast of Baghdad, unidentified assailants shot and killed an Iraqi nuclear scientist as he was on his way to work, witnesses said. Taleb Ibrahim al-Daher, a professor at Diyala University, was killed as he drove over a bridge on the Khrisan river. His car swerved and plummeted into the water.

In northern Iraq, insurgents set ablaze a major pipeline used to ship oil to the Turkish port of Ceyhan, a principal export route, an official with the North Oil CO. said. Firefighters were on the scene, 70 miles southwest of Kirkuk.

The Associated Press and FOX News' Bret Baier contributed to this report.