Two American soldiers appear to have been abducted in Iraq, defense officials said Thursday.

The men and their Humvee had been stationed at an observation posted near the town of Balad, north of Baghdad (search), when they went missing Wednesday night, according to Pentagon officials who spoke on condition of anonymity.

A search by Apache attack helicopters (search) was started as soon as they were noticed missing, one official said, declining to say how their absence was noticed.

"There is no site of them or their vehicle," he said.

Another official said some Iraqi people reported seeing them "somewhere else," meaning a distance from the observation post. He declined to say whether the Iraqis saw the men or the Humvee or both — or whether they were in the company of Iraqis — or any other details.

The incident comes amid an increase in attacks on occupying forces in Iraq. An American soldier and two Iraqi civilians were killed in other attacks Thursday and casualties have been reported almost daily. Six British troops were killed and eight wounded in attacks in southern Iraqi town Tuesday.

The Balad region, which is about 35 miles from Baghdad, also has been a trouble area.

Two weeks ago U.S. Central Command (search) reported that an organized group of attackers ambushed a two-tank patrol from the Army's 4th Infantry Division near there. In an ambush officials said was unusual in the number of assailants and the coordination of the attack, the assailants hid in a thicket of reeds, detonated a land mine, then launched rocket-propelled grenades at the tanks.

The resulting ground and air assault killed 27 Iraqi fighters.

News of the feared abductions came as a U.S. Marine was killed while responding to an ambush in which three other Americans were wounded Thursday. A bomb exploded on the airport road near the capital, killing a U.S. soldier and injuring another, the U.S. military said. And attackers threw grenades at a U.S. and Iraqi civilian convoy in west Baghdad, killing two Iraqi employees of the national electricity authority traveling in one car, U.S. soldiers and Iraqi police said. The convoy had U.S. Humvees at the front and the back and two Iraqi civilian vehicles in the middle.

A U.S. military spokesman in Iraq, Maj. William Thurmond, played down this week's violence as a "spike" and not a trend. Thurmond said the spate of ambushes could be a response to recent U.S. raids on Baath Party strongholds.

More than 700 people have been captured since officials started a roundup of Saddam loyalists and weapons in "Operation Desert Scorpion," launched June 15, the Pentagon has said.