In the first attack, a grenade hit a civilian Iraqi bus that was passing a 4th Infantry Division convoy near the town of Mushahidah, about 15 miles north of Baghdad.
At least two Americans were seriously wounded, said Capt. John Morgan, a U.S. military spokesman in Baghdad. The number of casualties on the bus was unknown. Soldiers returned fire "to protect the convoy and the civilian bus," U.S. Central Command said in a statement.
Also Sunday, assailants fired rocket-propelled grenades at a U.S. military convoy near Dujayl, 35 miles north of Baghdad, lightly wounding two soldiers, Morgan said.
U.S. troops have come under sporadic but sometimes deadly attacks in recent weeks, prompting the sweep into towns where support for Saddam lingers.
Monday was the second day of a forceful new operation called Desert Scorpion based on intelligence pinpointing opponents of the U.S.-led coalition in Iraq. It followed the expiration on Sunday of an amnesty program for people turning in heavy weapons.
In other developments:
The revelation could hurt Prime Minister Tony Blair's efforts to maintain support for the war, which has been undercut by the failure of weapons hunting teams to uncover any evidence of the massive stockpiles that the U.S. and British claimed Iraq possessed.
Those questions have fueled calls for Congressional hearings on the quality of prewar intelligence, which begin this week
On Sunday, the Republican chairman of the Senate Intelligence Committee, Pat Roberts of Kansas, said there could be open hearings and a public report, as Democrats have demanded.
Meanwhile, the U.S. Army's Psychological Operations unit is increasing radio appeals for Iraqis involved in weapons of mass destruction programs to surrender for trial, offering leniency for those who cooperate.
Coalition troops are having little trouble finding conventional weapons on their sweep through bastions of Iraqi resistance.
In Khaldiyah, U.S. commanders said they were acting on a tip from an Iraqi man captured after he and two other men fired rocket-propelled grenades Saturday night at a routine U.S. patrol near an abandoned Iraqi ammunition dump. The other two men escaped and the prisoner pointed to two homes he said the insurgents had been using as a hideout.
When military police entered the homes, they found only families and a few hundred rounds of pistol and assault rifle ammunition buried in the backyard of one of them.
In an old box used to transport artillery shells, the soldiers found strips of highly explosive cordite that had been emptied out of artillery shells.
Next door, soldiers found one pound of C4 explosives on the roof along with a detonator cord.
The explosives appeared to come from an Iraqi ammunition dump about 1,000 yards away across an open field. Soldiers scanning the field spotted about 50 crates of artillery shells and a place nearby where looters were taking off the explosive warheads, dumping out the cordite and taking away the brass shells to sell as scrap metal.
U.S. troops had armed local volunteers to guard the hundreds of ammunition bunkers, but they had apparently failed to protect it at night.
As the low-flying helicopters spotted more ammunition cases on the roofs of other homes, they directed the military police to raid those buildings. The soldiers arrested eight more men, seized more C4 explosives weapons and anti-tank weapons. They allowed each family to keep an assault rifle for home protection.