Iraqi Cabinet ministers allied to radical cleric Muqtada al-Sadr threatened Wednesday to quit the government to protest the prime minister's lack of support for a timetable for U.S. withdrawal, according to a statement.

Such a pullout by the very bloc that put Prime Minister Nouri al-Maliki in office could collapse his already perilously weak government. The threat comes two months into a U.S. effort to pacify Baghdad in order to give al-Maliki's government room to function.

Al-Sadr's political committee issued the statement a day after al-Maliki rejected an immediate U.S. troop withdrawal.

"We see no need for a withdrawal timetable. We are working as fast as we can," al-Maliki told reporters during his four-day trip to Japan, where he signed loan agreements for redevelopment projects in Iraq.

"To demand the departure of the troops is a democratic right and a right we respect. What governs the departure at the end of the day is how confident we are in the handover process," he said, adding that "achievements on the ground" would dictate how long American troops remain.

Al-Maliki spoke a day after tens of thousands of Iraqis took to the streets of two Shiite holy cities, on al-Sadr's orders, to protest the U.S. presence in their country. The rally marked the fourth anniversary of Baghdad's conquer by American forces.

"The Sadrist movement strongly rejects the statements of Prime Minister Nouri al-Maliki, in which he stood by the continued presence of occupation forces despite the will of the Iraqi people," said the statement, a copy of which was obtained by The Associated Press.

"The Sadrist movement is studying the option of withdrawing from the Iraqi government — a government that has not fulfilled its promises to the people," it said.

"We are serious about withdrawing," it added.

It would not be the first time the Sadrists, who hold six seats in the Cabinet, left al-Maliki's government.

Al-Sadr's ministers and 30 legislators boycotted the government and parliament for nearly two months to protest a November meeting between al-Maliki and President Bush in Jordan.

The statement expressed anger over the Baghdad security plan launched on Feb. 14, calling it "unfair." Iraqi and U.S. troops have been targeting members of al-Sadr's Mahdi Army militia, which has been blamed for sectarian killings.