U.S. forces in Saddam Hussein's hometown of Tikrit (search) arrested three men believed to be leaders of a guerrilla cell responsible for attacks against American troops and Iraqi civilians, and troops seized weapons and bomb-making gear, the U.S. military said Thursday.

"They are suspected members of a local (guerrilla) organization called Mohammed's Army and they received funding from the elements of the former regime," said Lt. Col. Steve Russell, commander of the 1st Battalion, 22nd Infantry Regiment of the 4th Infantry Division.

• Video: Raids Yield Suspects, Weapons

Meanwhile, the U.S. Champion Army Base in Ramadi, 60 miles west of Baghdad, was attacked by a suicide bomber Thursday, but no American soldiers died, the military said.

But a military source said there were injuries among U.S. troops when the bomber detonated his explosive charge just outside the base's gate.

The region around Ramadi and the nearby city of Fallujah is one of the most dangerous for coalition troops and sits in the Sunni Triangle, where the majority of U.S. deaths in hostile action have occurred. 

The area has been relatively quiet in recent weeks. Both Fallujah and Ramadi are policed by the 82nd Airborne Division and several units of the National Guard.

Also Thursday, Ghazi al-Talabani, director of the Northern Field Protection Force (search), which guards oil pipelines in northern Iraq, said an explosion set a pipeline ablaze, forcing officials to halt the flow.

The pipeline links the Beiji refinery in northern Iraq with the al-Doura refinery near Baghdad. A complex grid of pipelines move oil and natural gas throughout the region, and it was unclear how major the pipeline was.

Meanwhile, U.S. officials said Thursday that an Apache helicopter that crash-landed near the northern city of Mosul (search) on Wednesday might have been hit by ground fire while making a low pass over the area, which has been the site of escalating anti-occupation resistance.

Separately, two U.S. soldiers were killed Wednesday and four injured in two separate attacks in Mosul.

And in Samarra, another volatile city 60 miles north of Baghdad, two members of the U.S.-led paramilitary Civil Defense Corps (search) were killed overnight by unidentified gunmen while on patrol, witnesses said Thursday.

Chopper Downed

A military spokesman earlier said the chopper from the 101st Airborne Division (search) was forced to crash land because of mechanical failure and that the crew, who were uninjured, reported no ground fire. But a commander later said that he didn't know whether ground fire brought down the aircraft.

Troops guarding the site where the burned-out wreckage was still smoldering on Thursday morning said the chopper had been hit by enemy fire while flying over the area on a low-level patrol.

"The helicopter was shot down," one soldier said.

"We don't know what happened," Brig. Gen. Frank Helmick of the 101st said later. "It could have been a mechanical failure but again, we are looking at all possibilities."

If it is confirmed that the Apache was brought shot down, it would be the sixth military helicopter downed in six weeks.

Mosul, the predominantly Sunni Muslim city is home to many former soldiers and party loyalists of Saddam Hussein, was the site of the deadliest incident so far involving U.S. forces. On Nov. 17, two Black Hawk helicopters collided and crashed, killing 17 soldiers. Although military spokesmen initially insisted that the collision was the result of an accident, officers have since acknowledged that ground fire was the likely cause.

In Baghdad, guerrillas struck a U.S. Air Force C-17 transport plane on takeoff with a ground-fired missile, forcing it to return to the capital's international airport, a senior Pentagon official said Wednesday.

Raids Target Killers of Spaniards

Also on Wednesday, task force "All American" (search) -- paratroopers from 3rd Brigade, 82d Airborne Division with the assistance of Iraqi police -- conducted Operation Panther Squeeze (search), a series of 18 raids in Lutafiyah to kill or capture individuals believed to be responsible for the attack against Spanish forces on November 29.

During the raids, 15 primary targets were captured and 41 enemy personnel were taken for questioning. Those captured included cell leader Abu Abdullah, an intelligence officer, financier, and a doctor who treated terrorists so they can avoid treatment at local hospitals, and the actual attackers.

A vehicle was also confiscated that may have been used in the recent assassination of the Lutafiyah police chief.

In Madrid, Spanish Prime Minister Jose Maria Aznar announced the arrests of suspects in the slaying of seven Spanish intelligence agents in an ambush south of Baghdad. The U.S. Army said in a statement that 41 "enemy personnel," including the alleged leader of the cell that carried out the attack, were detained.

U.N. Secretary-General Kofi Annan cited the continued violence as he said Wednesday that Iraq is still too dangerous to reopen the U.N. office in Baghdad. His comments came after he appointed Ross Mountain of New Zealand as a replacement for the top envoy to Iraq, who was killed in an August suicide bombing along with 21 other people.

Most U.N. functions in Iraq will operate from a new regional base in Nicosia, Cyprus, with about 40 international staffers in place there by early 2004. There will be a smaller U.N. office in Amman, Jordan, Annan said in a 26-page report to the Security Council.

Mountain will temporarily take over Sergio Vieira de Mello's duties until a permanent replacement in named, probably early next year.

U.S. defense officials in Washington said 250 of the 700 Iraqi soldiers trained by the U.S.-led occupation authority have quit. The battalion completed a nine-week basic training course in October and was to be the core of a new Iraqi army.

It was uncertain exactly why a third abandoned their new jobs, though some had complained that the starting salary -- $60 a month for privates -- was too low, officials said.

The Associated Press contributed to this report.