BAGHDAD, (Reuters) – U.S. forces called in helicopter strikes during a clash with gunmen on Thursday in the city of Hilla and said they bombed a house in Basra, stepping up raids after days of relative calm in Iraq’s volatile south.

Shi’ite cleric Moqtada al-Sadr, whose followers fought Iraqi and U.S. forces last week, called for a “peaceful sit-in” in Baghdad on Friday to protest against bombings, arrests and vehicle bans that continue to seal off parts of the capital.

Police sources in the Shi’ite city of Hilla said five people had been killed in Thursday’s predawn clash, including four policemen.

In Basra, where last week’s fighting erupted when Prime Minister Nuri al-Maliki launched a crackdown on Sadr followers, a U.S. air strike late on Wednesday destroyed a house.

Reuters television pictures showed a woman’s body in the rubble and rescue workers searching for more dead. Police sources said at least three people had died including a mother, father and son, and three were seriously wounded. A U.S. military spokesman said the strike killed “one enemy”.

Iraq has been mostly calmer since Sunday, when cleric Sadr ordered his fighters off the streets after a week of fighting spread through the south and Shi’ite neighbourhoods of Baghdad.

The violence exposed a deep rift within Iraq’s majority Shi’ite community and served as a reminder of the country’s underlying instability after months of security improvements.

Hundreds died, making March the deadliest month for Iraqi civilians since last August, according to government figures.

Nevertheless, the chairman of the U.S. joint chiefs of staff, Admiral Mike Mullen, said Washington would not alter plans to withdraw about 20,000 troops by the end of July.

The top U.S. officials in Iraq, commander David Petraeus and ambassador Ryan Crocker, are due to testify to Congress next week. They are expected to recommend a pause in withdrawals after July to safeguard the past year’s improvements.

Sadr aide Hazem al-Araji said the cleric had called the sit-in to start after prayers on Friday to protest against “the siege of some neighbourhoods, the random arrests of Sadrists and bombing innocent civilians”.

“We will continue the sit-in until the bombing, besieging and arrests are halted,” he told Reuters.

Iraqi forces lifted a ban on vehicles in one neighbourhood, Kadhimiya, on Thursday but continue to seal off two of Sadr’s other strongholds in the capital, Shula and Sadr City.

Hilla was one of the southern towns that saw some of the heaviest fighting last week, including helicopter strikes called in by units made up of U.S. special forces and Iraqi troops.

The U.S. military said clashes resumed there on Thursday before dawn when gunmen opened fire on U.S. troops heading to detain a so-called “special groups” member, the U.S. term for rogue elements in Sadr’s Mehdi Army militia.

One vehicle was destroyed, several buildings were damaged and at least three civilians were wounded, a U.S. spokesman said. The spokesman for the main hospital in Hilla said six bodies had been brought in after the clash, and around 15 people were wounded, half of them civilians.

Basra, which controls both Iraq’s only major port and oilfields that provide 80 percent of its export income, has seen a turf war between rival Shi’ite political parties.

The government said its crackdown last week was an attempt to impose order in the lawless city. But Sadr’s followers and other opponents say it was a campaign by parties in Maliki’s government to suppress rivals ahead of local elections.