"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.

In the bombing Friday, the attacker detonated his explosives belt around noon as he waited in a line to be searched at an entrance to the mosque in Musayyib, 40 miles south of Baghdad, a police officer said on condition of anonymity because of security concerns.

Dr. Saad al-Janabi of the local hospital said 12 people died and 18 people were wounded. The police officer earlier said 11 people died, but al-Janabi said one of the injured died in the hospital.

The U.S. military said eight civilians were killed and 15 wounded.

Mahdi Rajab, owner of a nearby grocery store, said the force of the blast shattered windows.

"I went outside to see what happened and I saw dead bodies at the entrance of the mosque and worshippers were running away in panic," he said.

There was no claim of responsibility, but suicide bombings are associated with Sunni extremist groups. The U.S. military has warned Sunni insurgents are trying to provoke revenge attacks by Shiites in order to reignite sectarian warfare.

Under the security pact, U.S. forces will withdraw from Iraqi towns and cities by June 30 and the entire country by Jan. 1, 2012. Iraq will have strict oversight over the nearly 150,000 American troops now on the ground, representing a step toward full sovereignty for Iraq.