BAGHDAD, Iraq — Insurgents claiming to be linked to al-Qaida tried to overrun a U.S. Marine base on the Syrian border yesterday using gunmen, suicide car bombs and a firetruck loaded with explosives, U.S. and Iraqi officials said.

U.S. officials described the assault as the second time in less than two weeks that foreign insurgents have employed new tactics of massing an organized military-style offensive rather than staging smaller-scale bombings and attacks.

In another development, an American contractor believed to be working on an aid project was reported kidnapped in the Baghdad area, the U.S. Embassy said. Authorities released no other information, but soldiers stepped up searches of vehicles entering the heavily fortified Green Zone.

Yesterday's raid was on Camp Gannon, a U.S. base at Husaybah, a few yards from the Syrian border near the Euphrates River. U.S. Cobra attack helicopters fired on the insurgents to repel simultaneous attacks by suicide bombers and armed fighters, officials said. A second car bomb exploded 15 minutes after the first assault, "at the same entrance, while the soldiers were busy rescuing the wounded," Iraqi army Capt. Saad Abdul Fattah said.

The U.S. military said three Marines were wounded and at least three bombers were killed. Witnesses and a hospital spokesman reported that 10 to 15 people were killed, including foreign and Iraqi insurgents.

A statement posted on an Islamic Web site purportedly from a group known as al-Qaida in Iraq, led by Abu Musab al-Zarqawi, asserted responsibility for the attack.

Camp Gannon is an abandoned railway station that houses hundreds of Marines along a border long used by smugglers and other outlaws. Insurgents have operated openly in some towns along the border.

The suicide bombers, driving a firetruck, a pickup truck and one other vehicle, "attempted to breach the perimeter of Camp Gannon," the U.S. military said in a statement.

The bombs exploded prematurely, slightly damaging the camp defenses of concertina wire and barricades. A mosque and other surrounding buildings also sustained minor damage, the statement said.

Marines came under small-arms fire at the same time, the military said. A 25-year-old student who witnessed the attack said at least 40 Arab and Iraqi fighters took part in the assault.

Cobra attack helicopters fired on a vehicle carrying an unknown number of gunmen, destroying it, the military said.

The attack came nine days after al-Zarqawi's group asserted responsibility for an unsuccessful attempt to breach the walls of the U.S.-run Abu Ghraib prison in a concerted attack involving mortars, rockets, car bombs and ground fighters. U.S. officials said the two massed attacks were well-planned and mark a change in strategy for foreign groups that have been known for individual suicide bombings and kidnappings.

Al-Zarqawi's group appears to be trying now for a spectacular coup against U.S. forces in Iraq, in a bid to regain flagging popular support and momentum, a U.S. official in Baghdad said.

An Iraqi insurgent commander with al-Zarqawi's group who claimed he helped lead the Abu Ghraib assault recently said the movement had been scouting Anbar province in search of a U.S. base to attack with suicide bombers and heavy weapons. Yesterday's assault was in Anbar province.

Fighting also was reported yesterday in the town of Qaim a few miles inside the border. Al-Zarqawi's group claimed on its Web site to be in control of the town, and there were reports of several deaths. There was no indication that U.S. forces were involved.

Also yesterday, insurgents detonated a truck bomb in a crowded market in Samarra, targeting a U.S. military convoy on patrol.

At least three people were killed and more than 20 were wounded, including four U.S. soldiers, officials said.

The violence came as hundreds of Iraqi and American troops swept through a large section of Baghdad, raiding homes and arresting dozens of people suspected of links to the insurgency.

The raids provided the latest example of the growing role Iraqi forces play in combating the guerrillas. Iraq's new leaders, with the support of American officials, are working toward a transfer of security duties that would allow U.S.-led forces to draw down their numbers.

The raids, in Baghdad's al-Rashid district, were the biggest in weeks, involving about 500 Iraqi troops and 200 American soldiers, the military said.

The zone includes the Dora neighborhood in southern Baghdad, which over the past several months has seen scores of kidnappings, bombings and killings. Over the weekend, a Shiite cleric from Karbala and a police official from Najaf were killed in Dora.

The troops, moving on foot through the crowded streets, arrested 65 suspects.

In Kirkuk, about 180 miles north of Baghdad, gunmen opened fire late yesterday on a police patrol in the city of Kirkuk, wounding two of the officers, a police official said.

Attackers also placed a bomb in the undercarriage of a doctor's car, but the device exploded as the physician entered a Kirkuk store, sparing him but wounding two civilians, police said.

Information from the Chicago Tribune and The Associated Press is included in this report.