BAGHDAD, Iraq (CNN) -- The office of Muqtada al-Sadr accused Iraqi and U.S. forces of attacking Sadr City on Friday, just hours after the Shiite cleric called for calm in the wake of the assassination of one of his top aides in the southern city of Najaf.
Iraqi Sadrists march Friday in the Najaf funeral procession of Sayyed Riyadh al-Nuri, top aide to Muqtada al-Sadr.
Witnesses and media in the heavily Shiite Baghdad neighborhood of Sadr City, home to the cleric's power base in the capital, reported heavy fighting between U.S.-backed Iraqi troops and al-Sadr's Mehdi Army militia.
The witnesses said U.S. aircraft had been bombarding the area for hours, and media reported rockets slamming into houses and many casualties.
Witnesses and al-Sadr's office said mosques were making loudspeaker announcements about Mehdi Army attacks on U.S. military armored vehicles.
U.S. troops working in support of Iraqi soldiers killed two snipers, two other men firing rocket-propelled grenades and "multiple others from a nearby building where soldiers were taking RPG and machine gun fire," the U.S. military said in a statement.
At the same time -- about 9 p.m. -- at least six roadside bombs damaged vehicles in a U.S. Army convoy that was transporting barriers for a group of Iraqi Army soldiers establishing a checkpoint, the military said.
Afterward, the military said, the Iraqi and U.S. soldiers were attacked by small-arms, machine-gun and RPG fire from buildings overlooking the road.
The soldiers fired back at their attackers, killing at least four of them.
More explosions from the buildings indicated possible arms and munitions stored there, the military said. But the small-arms attack continued until the U.S. forces fired two rounds from an M1A2 Abrams tank, killing two more attackers.
Not long afterward, the U.S. Air Force, operating an unmanned aerial vehicle, fired a Hellfire missile at three men spotted setting roadside bombs, killing all three.
Maj. John Gossart, executive officer of the American unit involved, said that no U.S. or Iraqi troops were seriously hurt.
Earlier, al-Sadr issued remarks about the killing of Sayyed Riyadh al-Nuri, who was shot outside his house in Najaf's Adala neighborhood after returning from Friday prayers.
"The hands of the occupiers and their collaborators have treacherously reached our beloved martyr Sayyed Riyadh al-Nuri," al-Sadr wrote in a statement on the Web.
Al-Nuri is one of 17 people killed over 24 hours in airstrikes, fighting and attacks in areas wracked in recent weeks by fighting among Shiites.
The assassination prompted an immediate vehicle ban in the Shiite holy city of Najaf, anger among mourners and an intensification of fighting in Baghdad's Sadr City neighborhood.
Iraqi Prime Minister Nuri al-Maliki issued a statement deploring the killing and ordering an investigation.
Al-Sadr issued remarks about the killing in a statement on a Web site. Spokesman Sheikh Salah al-Obeidi emphasized that the cleric is not accusing anyone in particular of the killing but believes that the killers "are the ones who are following the occupiers' steps and don't want stability for the country."
But al-Obeidi called the killing an "act of provocation" after the "siege of Sadr City."
He was referring to the battles since Sunday involving members of al-Sadr's Mehdi Army militia and Iraqi security forces dominated by a rival Shiite political movement, the Islamic Supreme Council of Iraq.
That fighting started with an offensive in Basra and spread to other Shiite regions, including Sadr City and the Babil provincial capital of Hilla.
The al-Nuri assassination prompted officials to expand the daily curfew in Hilla. Police said a ban on all outside movement that usually begins at 11 p.m. and ends at 8 a.m. will instead start at 8:30 p.m.
Violence continued Friday in several places in Iraq.
Suicide bombings killed at least four people -- three of them police -- and wounded 15, officials said.
The first bombing was in Ramadi, the provincial capital of the predominantly Sunni Anbar province west of Baghdad, an Interior Ministry official said. At least three national police officers were killed and five wounded, the official said.
The second attack took place at a checkpoint about 20 km (12 miles) north of Baiji, according to police, who said the bomber and one other person were killed and 10 were wounded.
The casualties were members of a local Awakening Council who were manning the checkpoint, police said. The suicide bomber was driving a pickup carrying sheep.
Awakening Councils, or Sons of Iraq, are made up of Sunnis who have turned on al Qaeda in Iraq.
Also, at least three people were killed and five wounded in a mortar attack on Baghdad's Palestine Hotel, an Interior Ministry official said.
The Palestine Hotel -- across the Tigris River from the International Zone, the heavily guarded seat of U.S. power in Baghdad -- is in the path of many of the rockets and mortars aimed at the zone.
The U.S. military has blamed Iranian-backed Shiite militants for recent mortar and rocket attacks in Baghdad and International Zone, also known as the Green Zone.
Unmanned aerial vehicles targeted and killed six suspected insurgents in Basra on Friday and six "heavily armed criminals" in northeastern Baghdad on Thursday night, the U.S. military said. Watch the Baghdad drone attack »
The U.S. and Iraqi militaries have consistently said they have not been targeting specific groups in their recent battles in Shiite areas.
Iraqi and U.S. government officials say they differentiate between Mehdi Army members obeying al-Sadr's seven-month cease-fire pledge and "gangs," "criminals" or "outlaws" who aren't obeying al-Sadr's orders.
The intra-Shiite fighting in Iraq that has killed hundreds of people in the past two weeks has involved two main movements: members of the Mehdi Army militia loyal to al-Sadr, and Iraqi security forces dominated by the chief political rival of the Sadrists, the Islamic Supreme Council of Iraq.
Many of those security forces had been integrated into police and army units from the council's Badr Brigade militia. E-mail to a friend
CNN's Yousif Bassil and Jomana Karadsheh contributed to this report.