American troops and helicopters scoured the desert Thursday for two U.S. soldiers who were apparently abducted from an observation post north of Baghdad (search). Ambushes and hostile fire elsewhere in Iraq (search) killed two U.S. soldiers and two Iraqi civilians and wounded eight other Americans.

A day after a U.S. Marine was killed responding to an ambush on Americans, reports of attacks on U.S. troops appeared almost hourly — too frequent for military press officers to keep up with. Most of the information came from witnesses at the attack scenes.

Between Wednesday and Thursday, assailants blew up a U.S. military vehicle with a roadside bomb, dropped grenades from an overpass, destroyed a civilian SUV traveling with U.S. troops, demolished an oil pipeline and fired an apparent rocket-propelled grenade at a U.S. Army truck.

Officials played down the violence, but with shattered glass, blood stains and mangled vehicles littering the landscape, the upsurge in attacks is causing concern that the U.S.-led occupation of Iraq could be turning into a guerrilla war.

In the latest and most serious reported attacks:

— A member of a U.S. special operations (search) force was killed and eight were wounded Thursday morning by hostile fire southwest of Baghdad, the U.S. military said, giving no further details.

— A bomb exploded on the Baghdad airport road, killing a U.S. soldier and wounding another, the military said. The road — heavily used by U.S. forces — has been the scene of several attacks using trip wires dangling from overpasses or grenades tossed from bridges.

— In another ambush, assailants threw grenades at a U.S. and Iraqi civilian convoy in west Baghdad, killing two Iraqi employees of the national electricity authority, U.S. soldiers and Iraqi police said. The convoy had U.S. Humvees  (search) at the front and the back and two Iraqi civilian vehicles in the middle. The victims were traveling in the same car.

A military spokesman, Maj. William Thurmond, said the spate of ambushes could be a response to recent U.S. raids on Baath party (search) strongholds.

"There have been more attacks recently, but it's probably premature to say this is part of a pattern," Thurmond said. "We've kicked open the nests of some of these bad guys."

An Iraqi police official, Brig. Ahmed Khazem, called the ambushes "isolated actions ... carried out by individual mercenaries."

The Arab satellite station Al-Jazeera (search), however, aired statements Thursday from two previously unknown groups urging assaults on U.S.-led forces in Iraq.

One, by a group calling itself the Mujahedeen of the Victorious Sect (search), claimed responsibility for recent attacks and promised more. The other, by the Popular Resistance for the Liberation of Iraq (search), called for "revenge" against America.

Al-Jazeera said it could not verify the statements.

Two U.S. officials familiar with intelligence information said they had not previously heard of the groups issuing the statements and had no way to know whether they were credible.

Meanwhile, Pentagon officials said Thursday that two American soldiers were apparently abducted.

The men and their Humvee were stationed at an observation post near the town of Balad, north of Baghdad, when they were noticed missing Wednesday night, according to the Pentagon officials, speaking on condition of anonymity.

A search by Apache attack helicopters began immediately, one official said, declining to say how their absence was noticed.

Even before the latest violence began, U.S. intelligence officers had warned ground commanders to expect an increase in attacks against U.S. forces between June 25 and July 10. It was not clear on what intelligence the warning was based.

The U.S. military has blamed attacks on isolated remnants of Saddam Hussein's regime and his Sunni Muslim (search) followers, claiming there was no organized resistance.

But the attacks have now spread to Shiite (search) areas south of Baghdad that had been largely free of the violence plaguing the Sunni areas north and west of the capital — where Saddam had enjoyed a degree of support.

On Tuesday, gunmen furious over the killing of five civilians during a demonstration, allegedly at the hand of British troops, shot and killed six British troops in the southern town of Majar al-Kabir. A day later, an ambush wounded three Marines in Hillah, 45 miles south of Baghdad.

A Marine was killed and two were injured when their vehicle — part of a quick reaction force dispatched in response to the Hillah ambush — rolled over on the soft shoulder on the way to the scene.

The names of the American and Iraqi victims of the latest attacks were not immediately released. The killings raised the American death toll to 196 since the start of the war on March 20. At least 20 U.S. soldiers have died as the result of hostile fire since major combat was declared over in May.

Responding to the violence, U.S. forces pressed ahead with aggressive patrols throughout Iraq, conducting 1,185 day patrols and 975 night patrols, a U.S. military statement said. They also conducted 199 day patrols and 122 night patrols jointly with Iraqi police, the statement said.

U.S. soldiers in Khaldiyah, 40 miles west of Baghdad, raided three homes and arrested four suspects after an informant provided them the names of six men allegedly involved in ambushes against American forces.

Backed by Bradley fighting vehicles, a platoon of about 35 soldiers from A Company, 3rd Battalion, 7th Infantry Regiment of the 3rd Infantry Division rushed toward the homes.

An old women in one of the houses shouted "Dogs! Dogs!" at the troops.