BAGHDAD — Shiite cleric Muqtada al-Sadr threatened Tuesday to lift a seven-month freeze on his Mahdi Army militia if the Iraqi government does not halt attacks on his followers or set a timetable for a U.S. withdrawal.

The statement came from his office hours before the top two U.S. officials in Iraq — Gen. David Petraeus and Ambassador Ryan Crocker — planned to brief Congress on the situation in Iraq and prospects for drawing down American troops.

"I call on the Iraqi government, if it exists, to work for the protection of the Iraqi people, stop the bloodshed and the abuse of its honor," al-Sadr said. "If the public interest dictates the lifting of the freeze to achieve our goals, beliefs, religion, principles and patriotism we shall do that later."

U.S. and Iraqi troops stepped up their pressure on Mahdi Army militiamen in their Baghdad stronghold of Sadr City, where fighting broke out again early Tuesday after Iraqi units tried to enter the area, a police officer said.

Explosions could be heard across the capital, apparently coming from the neighborhood in eastern Baghdad. Low-flying jets circled the center of the capital several hours before sunrise.

The two sides were using small arms, rocket-propelled grenades and mortars, said the police officer, who spoke on condition of anonymity because he was not authorized to release the information to the media.

Fifteen civilians were wounded in the clashes, he said.

Using typical rhetoric to refer to U.S. forces, al-Sadr said the government should "protect the Iraqi people from the booby traps and American militias" and "demand the withdrawal of the occupier or a schedule for its withdrawal from our holy land."

Al-Sadr has extended an order made last year to his Mahdi Army fighters to stand down through August, but he warned he could lift the unilateral cease-fire if the Iraqi government fails to provide the public with security and stability.

Al-Sadr also called off a "million-strong" march planned for Wednesday in Baghdad after followers in Shiite areas south of Baghdad complained that Iraqi security forces prevented them from traveling to the capital.

"I call on the beloved Iraqi people who want to demonstrate against the occupation to postpone their outing for I fear for them and I want to spare their blood," he said in the statement. The demonstration was supposed to mark the 5th anniversary of the capture of Baghdad by invading U.S. forces.

Hassan al-Rubaie, a Sadrist lawmaker, said that now that the demonstration has been canceled the government should lift its siege on Sadr City and Shula, another Mahdi Army stronghold in Baghdad.

The cease-fire, which was first imposed for six months in late August and later extended for another six-month period, has been credited with helping bring down violence. But it has been under severe strain since fighting broke out last month after the government launched a crackdown on militia violence in the southern city of Basra.

"The freezing of the Imam Mahdi Army was beneficial to the Iraqis, but it will have to be lifted if it becomes a hindrance," al-Sadr's chief spokesman Salah al-Obeidi said after he read al-Sadr's statement during a news conference in Baghdad.