A roadside bomb killed 10 Marines and wounded 11 others on a foot patrol near Fallujah, the U.S. military announced Friday. It was the deadliest attack against American troops in four months.

The ambush occurred Thursday against Marines from Regimental Combat Team 8, based at Camp Lejeune, N.C. The Marine unit has suffered some of the highest casualties of the Iraq war.

The unit's latest losses were among 14 new deaths in Iraq announced by the military Friday. With at least 793 American lives lost since January, 2005 appears on track to become the deadliest year for the troops since the U.S.-led invasion in March 2003. There were 846 deaths in 2004, and 485 the previous year.

Altogether, at least 2,127 have died since the beginning of the war, according to an Associated Press count.

The attack in Fallujah came a day after President Bush outlined his strategy for victory in Iraq, and at a time when there are growing calls for an exit plan for U.S. troops.

Hours after the military announced its grim news, Al-Jazeera broadcast a videotape and statement in which the kidnappers of four Christian peace activists threatened to kill the hostages — two Canadians, an American and a Briton — unless all prisoners in U.S. and Iraqi detention centers are freed by Dec. 8.

The roadside bomb in Fallujah, the former insurgent headquarters west of the capital, was fashioned from several large artillery shells, the military said.

Sen. John Warner, chairman of the Senate Armed Services Committee, called the ambush a "very serious attack," saying "it appears that this group of Marines had collected — which is always a dangerous thing — in sort of one location."

The military statement said seven of the wounded later returned to duty and that the rest of the team was conducting "counterinsurgency operations throughout Fallujah and the surrounding area" to improve security for the Dec. 15 elections.

Names of the victims were not released pending notification of their families. The statement also did not give the precise location of the attack — the single deadliest against U.S. troops in Iraq since 14 Marines were killed Aug. 3 when a bomb destroyed their vehicle near Haditha, 140 miles northwest of Baghdad.

Maj. Cliff W. Gilmore, a spokesman at Camp Lejeune for the II Marine Expeditionary Force, said the victims of Thursday's ambush probably came from hometowns across the country rather than from one area.

"Here at Camp Lejeune, we pause, we stop, we feel it, and then we carry out with the mission," Gilmore said. "Those folks in Iraq, they probably didn't even have time to pause and think about it. ... The folks that are on patrol right now might not have even heard of it."

Also Friday, three U.S. soldiers from the 48th Brigade Combat Team were killed in a traffic accident south of Baghdad, and the military said an Army soldier assigned to the 2nd Marine Division died of wounds suffered the previous day when his vehicle was struck by a rocket in Ramadi, 70 miles west of the capital.

U.S. forces have stepped up military operations throughout the Sunni Arab regions west of Baghdad to cut off the flow of weapons, ammunition and foreign fighters entering the country from Syria and to reduce insurgent activity.

As part of that campaign, the U.S. military on Friday launched a new offensive — Operation Shank — in Ramadi, capital of the insurgent-ridden Anbar province. About 200 Iraqi army soldiers and 300 U.S. Marines were taking part in the offensive, the fifth in Ramadi since Nov. 16.

On Thursday, insurgents allowed a local AP Television News cameraman to film gunmen as they strolled briefly through empty streets, kneeled with their weapons at the ready and issued a declaration claiming they were "controlling the city." Ramadi has been the scene of repeated insurgent attacks but the U.S. military disputed claims the rebels control a significant area of the city.

Fallujah, located about 40 miles west of Baghdad, was an insurgent bastion until U.S. forces overran the city in November 2004 in the most intense urban combat of the Iraq war.

Since then, the U.S. military and the Iraqi government have been working to rebuild Fallujah and limit the return of insurgents, many of whom slipped out of the city during the siege and moved into nearby towns and villages outside the security cordon.

Regimental Combat Team 8 is a part of the II Marine Expeditionary Force. In the nearly three years since the war began, 147 Marines from II MEF have died in combat, according to 2nd Marine Division spokesman Lt. Barry Edwards. Regimental Combat Team 8 has been in Iraq since the beginning of February.

U.S. officials hope the operations will enable Sunni Arabs to vote in the parliamentary elections without fear of insurgent reprisals — which the Americans blame in large part for the Sunni boycott of the January balloting.

Washington hopes a big Sunni turnout will produce a government that can win the trust of the Sunnis, the backbone of the insurgency, and convince more of them to lay down their arms. That would hasten the day U.S. troops could go home.

However, many Sunni politicians fear that military operations so close to the election will have a negative impact by frightening voters away from the polls and deepening hostility to the Americans and their Iraqi partners.

A major Sunni clerical group, the Association of Muslim Scholars, appealed to the Arab League and human rights organizations to intervene with the Americans to stop "the massacres in Anbar." The Association is believed to have ties to some Sunni insurgent groups and is an outspoken critic of the American role in Iraq.

U.S. casualties have been increasing in recent weeks at a time of growing discontent within the United States over the Iraq conflict.

American commanders say they have been making gains in the war. On Thursday, Maj. Gen. Rick Lynch, a coalition operations officer, said suicide bombings declined to 23 in November because of successful military operations in the Euphrates River valley west of the capital.

However, U.S. and Iraqi officials have also predicted an increase in insurgent attacks as the election approaches. As part of security measures for the vote, Iraq's Interior Ministry has banned all non-Iraqi Arabs from entering the country, officials said Friday.

The decision was made Tuesday by Interior Minister Bayan Jabr, said two senior ministry officials who spoke on condition of anonymity because they are not authorized to speak to the media. They said the ban is expected to be lifted two days after the election.

On Friday, Jordan's airline prevented an Associated Press correspondent of Egyptian nationality from boarding a plane to Baghdad. Royal Jordanian officials said they had orders from Iraqi authorities.