Six British soldiers were killed in a police station in southern Iraq and eight were wounded in a nearby ambush Tuesday, marking the deadliest day of attacks on coalition forces since the fall of Saddam Hussein (search)'s regime.

The casualties were a shock to British troops occupying the largely Shiite south, which until now had been essentially free of the daily hit-and-run attacks plaguing American soldiers in central and western Iraq. British troops have felt so secure they have been patrolling the country's second-largest city, Basra (search), without flak jackets or helmets.

The U.S. military said insurgents had increased their attacks on American and British troops: 25 over a 24-hour period, including a firefight in Ramadi, west of Baghdad (search), that killed three Iraqis and wounded an American soldier.

The violence fueled concerns that Iraq is descending into a guerrilla war despite U.S. insistence that resistance is local, not centrally organized.

Defense Secretary Donald H. Rumsfeld, however, said coalition troops were succeeding in mopping up the resistance.

"They're making progress against the dead-enders who are harassing coalition forces," Rumsfeld said at the daily defense department briefing. "Just as they were unable to stop the coalition advance in Baghdad, the death squads will not stop our commitment to create stability and security in postwar Iraq."

The British casualties occurred in the town of Majar al-Kabir, about 180 miles southeast of Baghdad and just south of the city of Amarah.

Defense Secretary Geoffrey Hoon told Parliament in London that the British soldiers -- military police on a mission to train Iraqi police -- were apparently killed in a police station in the town.

Earlier, a British army spokesman in Basra said the soldiers were killed by Iraqi fire.

Elsewhere in the same town, a "large number" of Iraqi gunmen opened fire on a British patrol Tuesday with rocket-propelled grenades, heavy machine guns and rifles, Hoon said. The British returned fire, and one soldier was wounded in the fight.

A rapid reaction force, including Scimitar light tanks and a Chinook CH-47 helicopter, came to help the ground troops but also came under fire, Hoon said. Seven people on board the helicopter were wounded, three of them seriously, the government said.

Hoon said commanders were investigating whether the deaths and the ambush were connected.

Gen. Richard Myers, chairman of the Joint Chiefs of Staff, extended condolences to the families of the dead soldiers.

"These losses are a reminder that Iraq remains a dangerous place," Myers said at the Pentagon. "But we must continue to stand firm."

It was the deadliest day for coalition forces since May 19, when six U.S. Marines died, most in a helicopter crash and a vehicle accident.

The deadliest single attack on coalition forces came on March 23, the early days of the U.S.-led invasion, when Iraqis opened fire on a U.S. Army maintenance convoy near the southern town of Nasiriyah, killing 11 soldiers.

At least 18 U.S. soldiers have been killed in Iraqi attacks since May 1, when major combat was declared over. Most of the attacks have occurred in the belt of central and western Iraq dominated by Sunni Muslims, Saddam's strongest supporters.

Saddam loyalists, Sunnis and ex-army soldiers are suspected in the attacks. The Shiite-dominated south has been largely peaceful since the regime's fall. The Muslim sect was long repressed by Saddam and rose up in some areas as coalition forces invaded the country in March. Shiites have since assumed leadership roles in many regions and moved to restore order.

Thus the British have not seen major violence for weeks.

"It's normally very quiet down here," said British Army Lt. Col. Ronnie McCourt, in Basra. "We've been here nearly two months now and this is the first time people have been deliberately, consciously shooting at us."

British Army Capt. Dennis Abbott insisted the attacks "in no way reflects the general security situation" in British-controlled areas.

Forty-two British troops have died -- 19 in accidents -- since the war began March 20. Britain had suffered no confirmed combat deaths since April 6.

In other attacks Monday and Tuesday, Iraqi insurgents fired rocket-propelled grenades at U.S. troops in at least three towns in western Iraq. In Baghdad, guerrillas fired a grenade near the headquarters of the U.S. administration, causing no injuries.

Late Monday, insurgents fired a rocket-propelled grenade at the mayor's office in Fallujah, 35 miles west of Baghdad -- the latest in a series of attacks against people seen as cooperating with the U.S. occupation.

U.S. troops shot and killed one of the ambushers, U.S. military officers said. But local residents at the scene said the man killed was not involved in the attack and was caught in the crossfire.

Meanwhile, Rumsfeld said there is no evidence that senior Iraqi leaders were among those killed in a U.S. attack a week ago near the Syrian border.

Initial news reports about the attack said Saddam or his sons were thought by U.S. intelligence to have been in the convoy, which was destroyed by U.S. air and ground forces.

An undetermined number of people in the convoy were killed in the raid. The attack left buildings and vehicles burnt and casualties on both sides of the Iraqi-Syrian border. At least five Syrian border guards were wounded, three of them treated by the Americans. The Syrians remained in U.S. custody.

Syria has kept a strict silence about the attack -- apparently seeking to avoid a confrontation with the United States that could hurt already tense relations. Syrian officials refused comment, and state-run television, newspapers and radio did not mention the clash.