BAGHDAD — A suicide bomber blew himself up among worshippers waiting to be searched outside a mosque run by followers of anti-American Shiite cleric Muqtada al-Sadr on Friday, killing at least 12 people, Iraqi officials said.

The blast in Musayyib, south of Baghdad, occurred a day after Iraqi lawmakers approved a security pact with the United States that will allow U.S. forces to stay in Iraq for three more years.

Proponents of the deal, which awaits the expected ratification by the three-member presidency, say the Americans are still needed because Iraqi forces aren't ready to take over security on their own despite a sharp drop in violence since last year.

The U.S. military handed responsibility for security in Babil province, where the suicide bombing occurred Friday, to Iraqi forces last month.

The security pact was backed by the ruling coalition's Shiite and Kurdish blocs and the largest Sunni Arab bloc, which wanted concessions for supporting the deal. But al-Sadr, who commands a 30-seat bloc in the 275-seat parliament, rejected the pact and said U.S. troops should withdraw immediately.

A key aide to al-Sadr linked Friday's bombing to the agreement and warned that the American presence can only to more violence. He appeared to be suggesting that U.S. forces are a source of instability, rather than part of the solution to the Iraqi conflict.

"The explosion that took place today near a Shiite mosque in Musayyib town is one of the consequences of the security agreement," Sheik Abdul-Hadi Al-Mohammadawi said during a sermon in the Sadrist stronghold of Kufa. "The Iraqi government cannot survive without the U.S. presence and as long as the Americans remain here, Iraq will be still a battlefield."

Al-Sadr called for peaceful protests and urged followers to close his offices and affiliated institutions for three days to show opposition to the pact, according to the statement read by his spokesman, Sheik Salah al-Obeidi.

"We offer our condolences to the Iraqi people over this calamity that has fallen upon them with the signing of the agreement of humiliation and indignity," the statement said.

Hundreds of al-Sadr's supporters demonstrated against the pact in Sadr City in eastern Baghdad, a stronghold of the cleric.

A cease-fire order by the Shiite cleric, who is believed to be in Iran, has been a key factor in the drop in violence over the past year. His militia had also been heavily targeted in U.S. and Iraqi operations.

In the southern city of Najaf, a Shiite cleric whose political party backed the security agreement disputed claims that Iraq was forced to accept the pact against its will.

"The approval of the security agreement was an Iraqi decision free of any external pressure and it was done through accord among Shiites, Sunni Arabs and Kurds," said cleric Sadralddin al-Qubanji, a member of Iraq's largest Shiite political party, the Supreme Islamic Iraqi Council.