BAGHDAD, Iraq,— Iraqi gunmen killed six British soldiers today in southern Iraq as violence and unrest spread to areas of the country that had been largely quiet since the war ended almost two months ago. The ambush of British forces, near Amara, on the Tigris River about 90 miles north of Basra, came in two apparently coordinated attacks, the first on a patrol of paratroopers in two vehicles, the second on military police officers on a mission to train Iraqi officers. The killings, coming in addition to fatal confrontations in two towns west of Baghdad today, made for one of the deadliest days for American and British forces since the fall of Saddam Hussein's government on April 9. But, speaking in London, Geoff Hoon, the defense secretary, said, "I would certainly caution against reaching any wider conclusions about the overall security situation in southern Iraq."

Still, the attacks in the largely Shiite south, which has been relatively quiet even as American forces have come under fire in central and western Iraq, suggested that few parts of the country are secure. There have been at least two dozen attacks on British and American forces already this week.

The intensity of the attacks has raised the question of the extent to which they might be coordinated by forces still loyal to Mr. Hussein, or at least by Iraqis opposed to the large American presence here.

At a news conference today at the Pentagon, Gen. Richard B. Myers, the chairman of the Joint Chiefs of Staff, indicated that commanders were reassessing their view about that opposition, previously portrayed as sporadic and disorganized. He said the American view now was that it was "uncertain" whether the attacks were being organized.

Military officials said earlier this month that they did not know whether former Baath Party members were paying Iraqis to fight the Americans, or whether the attacks were being instigated by foreign Arabs.

What appeared certain, the officials said then, was that some attacks were premeditated and involved cooperation among small groups of fighters, including a system of signaling the presence of American forces. 

Whatever the coordination, the frequency appears to be on the rise. 

Three Iraqis were killed today at a skirmish at a checkpoint in Ramadi, about 60 miles west of Baghdad, and one American soldier was injured. 

In Baghdad, three Iraqis were killed and at least four were wounded on Monday when an American military convoy began shooting in a residential neighborhood after a gun was fired during an argument between two Iraqis, residents said.

Early this morning, an Iraqi man was shot dead by American troops after launching a grenade at an electricity substation in Falluja, also to the west of Baghdad.

At least 18 American soldiers have been killed by Iraqis since May 1, when major combat ended. The attacks have been concentrated in what is sometimes called the Sunni Triangle, an area of central and western Iraq where the Sunni Muslim population and Mr. Hussein's support were concentrated.

By contrast, the British sector in the south had been quiet. Troops had taken to patrolling Basra, the largest southern city, without helmets or flak jackets.

Details of the attacks were scarce, but Mr. Hoon said they began in the small town of Al Majar al Kabir when paratroopers in two vehicles came under fire from "a large number of Iraqi gunmen" using rocket-propelled grenades, heavy machine-guns and rifles.

A Chinook CH-47 helicopter was sent to provide assistance, and it too came under fire. One soldier on the ground and seven in the helicopter were wounded, Mr. Hoon said.

In a second incident later in the day, six military policemen on a mission to train Iraqi policemen were killed, possibly at the local police station, Mr. Hoon said. Both incidents are being investigated.

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

The area around Amara, where the attack occurred, is a Shiite stronghold. The local population was generally opposed to Mr. Hussein, and by the time American marines reached the town in early April, local Shiites had seized control. Mr. Hussein, a Sunni, repressed Iraq's Shiites, often with great brutality. Shiites make up about 60 percent of the population.

In the center of the country, where Mr. Hussein's support was strong, regular ambushes on Americans continued over the last 24 hours.

Around 9:30 tonight, one of two military police Humvees traveling east on the main highway in the Madhiq neighborhood of Ramadi was hit by a single rocket-propelled grenade, witnesses said. The Americans involved were from the 115th Military Police Battalion, a National Guard unit from Rhode Island, according to soldiers from the Army's Third Infantry Division assigned to the area. About 10:15 the damaged Humvee and its three wounded occupants showed up at the field clinic of the Third Infantry, Second Brigade, 464th Armored Unit, where they were treated by Lt. Monica Casmaer, a physician's assistant. The M.P.'s survived the attack, she said, because the grenade turned out to be a dud.

"They got really lucky," Lieutenant Casmaer said, sitting in a tent at a rail depot that serves as her home and the field hospital. "If the grenade had exploded, they would be dead. It was a perfect shot."

The M.P.'s told her that the grenade had hit just above the driver's side door and ripped a furrow through the roof. The Humvee's gunner suffered shrapnel wounds to his face and neck and serious injuries to his hands.

At about 1:30 a.m. today, soldiers from the Third Infantry, Second Brigade, Company A killed a man who fired a grenade at a power substation in Falluja, according to Capt. Ronny Johnson of the Second Brigade.

He said insurgents regularly attack the substation, but he insisted that assaults on his men had become less frequent over the past few weeks. No soldiers were injured, and the substation was not damaged. The grenade launcher was found in the dead Iraqi's possession.

"He shot, he missed," said Captain Johnson. "He died."

The frequency of attacks on Americans has raised fears among many Iraqis that the military could take a more heavy-handed approach to them in retaliation, or that nervous soldiers might open fire too readily.

That may have been what happened in an eastern neighborhood of the capital called New Baghdad, according to accounts from residents. 

Around 5:30 p.m. Monday, two men were arguing on the street when one pulled a gun and fired a shot into the air, residents and shopkeepers said. At that moment, a convoy of American Army trucks passed by.

Residents said the soldiers began shooting indiscriminately, killing three people and wounding at least four. As the soldiers continued firing, they blasted out shop and car windows for a three-block stretch on the busy street, witnesses said.

A United States military spokesman in Baghdad said he had no record of the incident but would look into the reports.