Radical Iraqi Shi'ite cleric Moqtada al-Sadr is urging the Iraqi government to reject a deal that would allow U.S. troops to remain in Iraq.
In a statement issued Wednesday, al-Sadr said he would support Prime Minister Nouri al-Maliki's government "publicly and politically" if it does not sign the accord.
Al-Sadr opposes the presence of American troops in Iraq, and until a recent cease-fire his Mahdi Army militia clashed frequently with U.S. troops.
U.S. and Iraqi officials have been negotiating an agreement that would govern the presence of U.S. troops in the country once the United Nations mandate expires at the end of the year.
Meanwhile, Iraqi police say a suicide bomber has killed three policemen in an attack near the northern city of Mosul.
Police say the bomber rammed his car into a police station Thursday. Officials say four policemen were also wounded.
Elsewhere, Iraqi soldiers and police are leading a security offensive against insurgents in the eastern province of Diyala.
Iraq's defense ministry said Wednesday that security forces arrested at least 35 suspected militants on the second day of the offensive.
U.S. forces are providing support for the crackdown, which follows similar operations in the provinces of Basra, Maysan and Nineveh this year.
In other developments, the Reuters news agency said Thursday that the U.S. military has detained one of its cameramen in Iraq.
U.S. troops detained Ali al-Mashhadani, who also works for the BBC, on Saturday when he went to the U.S.-protected Green Zone in Baghdad to get a U.S. military press card.
A U.S. military spokesman tells the BBC that al-Mashhadani was detained because he is a "threat to the security of Iraq and coalition forces."
Both Reuters and the BBC have called for his release.
Some information for this report was provided by AFP.