Ambushers fired rocket-propelled grenades at two U.S. military convoys Sunday, in separate attacks that wounded at least four Americans, two of them seriously, a U.S. military spokesman said Monday.

In the first attack, a grenade hit a civilian Iraqi bus that was passing a 4th Infantry Division (search) convoy near the town of Mushahidah, about 15 miles north of Baghdad. A U.S. military statement said the number of casualties on the bus were unknown.

At least two Americans were seriously wounded in that attack, said Capt. John Morgan, a U.S. military spokesman in Baghdad (search).

Soldiers returned fire "to protect the convoy and the civilian bus," said the statement from U.S. Central Command (search).

Also Sunday, assailants fired rocket-propelled grenades at a U.S. military convoy near Dujayl, a town 35 miles north of Baghdad, lightly wounding two soldiers, Morgan said.

"The convoy returned fire, and the attackers fled the area. A quick reactionary force was dispatched to provide security for the convoy and pursue the attackers," the Centcom statement said.

The incidents were the latest in a series of attacks against U.S. occupation forces in Iraq. In response, the military has begun large-scale sweeps around the country in search of insurgents and weapons.

In the second attack, a grocery store owner said he heard gunfire, ran outside and saw that the last truck in a 15-vehicle convoy had been hit. Qusai Taha, 33, said he saw two American soldiers, apparently wounded, being taken out of the truck.

Later, Taha said, two Iraqis arrived at the scene on a motorbike and set the vehicle ablaze. Other witnesses in the area gave similar accounts. U.S. soldiers were working Monday to remove the burned out vehicle.