BAGHDAD - A cease-fire critical to the improved security situation in Iraq appeared to unravel Monday when a militia loyal to radical Shiite Muslim cleric Muqtada al Sadr began shutting down neighborhoods in west Baghdad and issuing demands of the central government.

Simultaneously, in the strategic southern port city of Basra, where Sadr's Mahdi militia is in control, the Iraqi government launched a crackdown in the face of warnings by Sadr's followers that they'll fight government forces if any Sadrists are detained. By 1 a.m. Arab satellite news channels reported clashes between the Mahdi Army and police in Basra.

The freeze on offensive activity by Sadr's Mahdi Army has been a major factor behind the recent drop in violence in Iraq, and there were fears that the confrontation that's erupted in Baghdad and Basra could end the lull in attacks, assassinations, kidnappings and bombings.

As the U.S. military recorded its 4,000th death in Iraq, U.S. officials in Baghdad warned again Monday that drawing down troops too quickly could collapse Iraq's fragile security situation.

Pentagon officials said that military leaders are watching for any signs of backsliding as they consider whether to keep drawing down troops below pre-surge levels.

President Bush spoke about the death toll, saying, "One day, people will look back at this moment in history and say, 'Thank God there were courageous people willing to serve, because they laid the foundations for peace for generations to come.' "

Even as he spoke, the situation on the ground was rapidly worsening.

On Sunday, a barrage of at least 17 rockets hit the heavily fortified Green Zone and surrounding neighborhoods, where both the U.S. and Iraqi government headquarters are housed, according to police. Most of them were launched from the outskirts of Sadr City and Bayaa, both Mahdi Army-controlled neighborhoods.

On Monday, the Sadrists all but shut down the neighborhoods they control on the west bank of Baghdad. Gunmen went to stores and ordered them to close as militiamen stood in the streets. Mosques used their loudspeakers to urge people to come forward and join the protest.

Fliers were distributed with the Sadrists' three demands of the Iraqi government: to release detainees, stop targeting Sadrist members and apologize to the families and the tribal sheiks of the men.

The Iraqi security forces issued a statement promising to deal with those who terrorized shopkeepers and students.

"It's an open sit-in until the government responds to our demands. If the government doesn't respond, we will have our own procedures," said Hamdallah al Rikabi, the head of the Sadr offices in Karkh, in western Baghdad.

In the southern port city of Basra, where Shiite groups are battling for power, the Mahdi Army is the most feared force. The British military pulled out of the city late last year, leaving the city in the militia's hands.

The Iraqi government announced a three-day security plan, beginning 5 p.m. Tuesday, to seal Basra off from other governorates and countries, shut down schools and all institutes of education and ban vehicles from entering the province. Prime Minister Nouri al Maliki, as well as the ministers of defense and interior, were in Basra on Monday.

Since Sadr froze his militia on Aug. 29 and renewed the freeze in February, militia members and Sadrists have railed against the government for targeting and detaining their members. In Basra, Sadr's office rejected the security plan and warned that it'll react if attacked or if Iraqi forces detain more Sadrists.

As Shiite violence rises, U.S. troop deaths also appear to be rising in places such as Baghdad, where the American military is thinning out its presence as part of its drawdown of five brigades. Attacks against civilians in the capital are rising, according to statistics compiled by McClatchy. Next week, the U.S. will finish pulling out the second of five surge brigades. As part of the drawdown, the military has moved battalions out of Baghdad toward more violent areas such as the northern city of Mosul and Iraq's northeastern Diyala province.

As the troop presence has shifted, so has the violence. For the first time since January, a majority of U.S. troops were killed in Baghdad, not in outlying northern provinces. Indeed, the U.S. military reached the death of its 4,000th soldier in Iraq on Sunday, when four U.S. soldiers were killed in southern Baghdad.

So far, this month, 27 soldiers have been killed in Iraq. Of those, 16, or 59 percent, died in Baghdad. In January, 25 percent of U.S. deaths happened in Baghdad, or 10 of 40.

Civilian casualties in Baghdad are also on the rise, according to a McClatchy count. After a record low through November, when at least 76 people were killed and 306 were injured, the deaths began to rise. In December, it crept up to 88 people killed, in January 100 and in February 172. As of March 24, at least 149 people were killed and 448 were injured.

Youssef reported from Washington. McClatchy special correspondents Laith Hammoudi reported from Baghdad and Ali al Basri reported from Basra.

© McClatchy Newspapers 2008