Early Thursday, gunmen attacked a Sunni mosque in the southern Baghdad district of Saidiya, sparking an hour-long clash before dawn with mosque guards and residents.

There were no casualties, but the walls of the mosque and nearby houses were damaged, police 1st. Lt. Thair Mahmoud said.

In the nearby Um al-Maalif district, gunmen killed two Sadrist militiamen in a drive-by shooting, police said. Elsewhere, the bodies of two al-Sadr loyalists were found late Wednesday.

The interim government blamed Sunni Arab insurgents for fierce clashes that erupted earlier this week in another Sunni area of the capital and underlined the deep distrust between the country's communities.

U.S. officials said the violence broke out Monday when attackers fired on Iraqi army patrols and a joint U.S.-Iraqi checkpoint in the northern district of Azamiyah. At least 13 people were killed before calm was restored Tuesday.

But Azamiyah residents said they took up arms when Shiite militias and commandos of the Interior Ministry moved into the area. Many Sunnis consider those groups little more than death squads.

In a statement late Wednesday, the prime minister's office denied any ministry forces were involved, and said three insurgent groups provoked the clashes by purporting to be Shiite militiamen and Interior Ministry commandos.

The statement identified the three insurgent groups as the Islamic Army of Iraq, the 1920 Revolution Brigades and al-Qaida in Iraq.

According to the statement, insurgents "have received orders to send elements to Baghdad for armed displays and to destabilize the city because of its political, demographic and media importance."