A U.S. soldier, meanwhile, was killed and another was wounded in a roadside bombing Saturday night, the military said.
The attackers who fired on the bus in Baghdad also detonated explosives in it, killing the driver and injuring another woman. Lt. Ali Omran of al-Dora police station said the women were working for the Americans, but he did not specify their jobs.
Early Sunday, an Iraqi woman working with U.S. troops as a translator was killed and another was critically injured when gunmen broke into their houses in Mahmoudiyah, said Dawood al-Taee, director of the city's hospital.
The civilian killings were part of a strategy by insurgents to deter cooperation between Iraqis and the coalition that plans to hand over some sovereignty June 30.
In other recent developments:
In the southern city of Nasiriyah, forces loyal to cleric Muqtada al-Sadr's fought Italian troops at two bridges across the Euphrates river on Sunday. Two al-Sadr militiamen were killed and 20 were injured, residents said.
Six Italian soldiers were slightly wounded, said Maj. Antonio Sottile, spokesman for Italian troops in Iraq. Militiamen were shooting from a civilian hospital, but Italian troops did not fire toward the facility to avoid civilian deaths, he said.
The ANSA news agency said a convoy transporting the Italian official in charge of Nasiriyah, Barbara Contini, came under attack as it neared coalition headquarters, but that she was uninjured. Two carabinieri paramilitary police were injured, the agency said, citing military sources.
Most civilian staffers of the coalition were evacuated from their Nasiriyah headquarters to a military base because of threats from fighters loyal to al-Sadr, the anti-American cleric who launched an uprising against the occupation last month and faces an arrest warrant in the murder of a rival moderate cleric last year.
A coalition official, Andrea Angeli, said only two civilians remained in the coalition headquarters, which was attacked Friday by al-Sadr militiamen who were pushed back by Italian forces.
The bomb that killed the U.S. soldier and wounded another exploded alongside their vehicle in Baghdad on Saturday night, the U.S. Army reported Sunday.
The death brought to 776 the number of U.S. service members who have died since the start of military operations in Iraq last year. Of those, 566 died from hostile action and 210 died of non-hostile causes.
The coalition is trying to disband al-Sadr's army and sideline its radical leadership before handing power to a new Iraqi government. U.S. forces and al-Sadr fighters battled in recent days in the southern holy cities of Najaf and Karbala, although both were relatively quiet Sunday.
Coalition forces guarding large quantities of captured arms and explosives at Karbala's Mukhaiyam mosque came under mortar fire overnight, said Lt. Col. Robert Strzelecki, spokesman for the Polish-led multinational force in south-central Iraq. There were no casualties.
Earlier in the week, coalition troops drove out insurgents who were using the mosque as a base of operations.
Apparent gunfire slightly damaged one of Shia Islam's holiest shrines in Najaf on Friday, prompting calls for revenge against the Americans and even suicide attacks.
On Sunday, Iran's supreme leader accused the United States of damaging the shrine and called U.S. actions "shameless" and "foolish."
"Muslims can't tolerate the shameless incursion of American forces into sacred places," Ayatollah Ali Khamenei was quoted as saying by the official Islamic Republic News Agency.
The U.S. military has said al-Sadr's al-Mahdi Army was probably responsible.