The truce could end violence that has killed several hundred people, trapped the 2 million residents of Sadr City in a battle zone and prompted aid workers to warn of a humanitarian crisis.

But it is unclear how much control the anti-American Sadr has over many of the militiamen who claim allegiance to him in Sadr City, stronghold of his Mehdi Army militia.

"Prime Minister Nuri al-Maliki has approved this agreement," government spokesman Ali al-Dabbagh said. "The Iraqi government calls on all parties to commit to this deal, to be calm and show self-restraint."

The U.S. military declined to make any immediate comment.

Dabbagh said the agreement called for militiamen to hand in their medium and heavy weapons.

He did not elaborate but this would include rocket and mortar launchers, which have been used to fire hundreds of shells at the Green Zone government and diplomatic compound since Maliki ordered a crackdown on militias in late March.

Sadr spokesman Salah al-Ubaidi told Reuters the deal had been made through the Sadr movement's bloc in parliament and the ruling Shi'ite alliance.

He said he expected the pact to take effect either on Saturday night or Sunday with a total halt to all Iraqi military activity for four days.

But much will depend on the militiamen who have been roaming the teeming streets of Sadr City.

U.S. helicopters have been hovering over Sadr City 24 hours a day, hunting rocket and mortar crews. It was unclear if Maliki had ordered the U.S. military to stop offensive operations.

"The prime minister will decide whether there is a need for U.S. forces (in Sadr City)," Dabbagh said.

Bahaa al-Araji, a senior parliamentarian from Sadr's movement, said the faction wanted no U.S. troops there.

"We accept Iraqi security forces can enter the city but we want no foreign forces," he said. Most U.S. ground troops have stayed in an area around the southern portion of the slum.

RESIDENTS UNDER FIRE

Maliki, himself a Shi'ite, has stressed his crackdown on militias is about restoring order. Dabbagh said the government was not targeting the Sadrist movement, but "outlaws."

Gunmen have been battling U.S. and Iraqi forces nearly every night in Sadr City for seven weeks, making life a misery for the largely poor Shi'ite community there. Several thousand people have fled but most have been holed up in their homes.

Ubaidi said the agreement called for aid to be delivered to residents and roads opened. After the four-day ceasefire, Iraqi forces could enter Sadr City and detain anyone they wanted if they had an arrest warrant, he said.

Asked if Sadr's supporters would adhere to the agreement, Ubaidi said: "I expect they will. But look, the government has made promises before, but not fulfilled these promises. This may have an impact on the fighters."

Ubaidi said he expected Sadr to issue a statement ordering a halt to fighting. Sadr is believed to be in Iran taking advanced Islamic studies and has not been seen in public for a year.

Ali al-Adeeb, a member of Maliki's Dawa party, told Reuters there had not been any discussion about dissolving the Mehdi Army, which the prime minister has demanded.

Maliki has threatened to ban the Sadr movement from local elections in October unless the cleric disbands the Mehdi Army, which was blamed for stoking sectarian chaos in 2006 and 2007.

Sadr threatened last month to formally scrap a truce he imposed on the Mehdi Army in August unless the government halted its crackdown. A few weeks later he told the militia to observe the truce -- which has at times seemed irrelevant -- leaving Iraqis guessing over his true intentions.

The cleric originally declared the truce in August to weed out rogue elements of the militia.