The government said it would not try to block the April 9 march if it was peaceful, though Prime Minister Nuri al-Maliki, who ordered a crackdown on militia in the southern city of Basra last week, threatened more strikes against Sadr's strongholds.

A statement released by Sadr's office in the holy city of Najaf called on Iraqis of all sects to descend on the southern city, site of annual Shi'ite pilgrimages that attract hundreds of thousands of worshippers.

"The time has come to express your rejections and raise your voices loud against the unjust occupier and enemy of nations and humanity, and against the horrible massacres committed by the occupier against our honorable people," it said.

The demonstration, called for the fifth anniversary of the fall of Baghdad on Wednesday, raises the prospect of unrest coinciding with a politically sensitive progress report to Congress by the top U.S. officials in Iraq.

"If his intention is to get a whole lot of people together and go and make trouble in Najaf, I don't think that is going to be very popular," U.S. ambassador Ryan Crocker told a briefing.

U.S. forces called in helicopter strikes during a clash with suspected Sadr gunmen on Thursday in the city of Hilla and bombed a house in Basra overnight, after days of relative calm that followed a truce Sadr announced on Sunday.

The truce ended six days of fighting that spread through southern Iraq and Baghdad.

Officially, the Iraqi government is sanguine about the march. Interior Ministry spokesman Major-General Abdul-Karim Khalaf told Reuters: "The right to hold a peaceful demonstration and express opinions is guaranteed by the constitution."

But Maliki has been uncompromising toward the Sadrists, fellow Shi'ites who helped install him in power in 2006 but broke with the government last year.

The prime minister told reporters the Basra crackdown could be repeated elsewhere, listing the Shula and Sadr City districts, Sadr strongholds in the capital.

"Basra was a prisoner and now it has been freed," Maliki said. "Other cities need the same battle, and also Baghdad in areas where people are still in the hands of these gangsters."

A senior member of Sadr's bloc in parliament, Bahaa al-Araji, said the prime minister "must stop playing with fire, or the Sadr bloc and the Mehdi Army are ready for this battle, a crucial battle".

In a later statement, Sadr accused elements in the security forces of continuing to arrest his followers.

"We advise everybody to end the disturbances and to stop the shedding of blood and the arrests immediately," he said.

MOBILISATION

Sadr has millions of followers and was able to summon tens of thousands of people onto the streets of Baghdad for demonstrations during last week's fighting.

The cleric also 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 were killed in Thursday's predawn clash and helicopter strike, including four policemen. U.S. forces said the fighting erupted when gunmen fired on them as they attempted an arrest.

A U.S. military spokesman said an air strike in Basra killed "one enemy" late on Wednesday.

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.

Last week's violence exposed a deep rift within Iraq's majority Shi'ite community and served as a reminder of the instability after months of security improvements.

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

Crocker and General David Petraeus, the top U.S. civilian and military officials in Iraq, will report to Congress next week and are expected to recommend a pause in U.S. troop cuts after July to safeguard the past year's improvements.

(Additional reporting by Khaled Farhan in Najaf, Aseel Kami, Dean Yates, Khalid Al-Ansary and Waleed Ibrahim in Baghdad, and Aref Mohammed in Basra; editing by Andrew Roche)