BAGHDAD — Roadside bombs killed the top Shiite official in a volatile area south of Baghdad and an anti-Al Qaeda Sunni sheik further north yesterday as internal power struggles within both Islamic sects threaten to complicate American efforts to stabilize the country.

Car bombs, meanwhile, struck Iraqi civilians in Baghdad and the northern city of Tal Afar, with at least 31 people killed or found dead nationwide, according to police reports.

Abbas Hassan Hamza, a political moderate and the top official in the Iskandariyah district, was killed by a bomb that struck his convoy while he was going to work, a police officer said. Four of his bodyguards also were killed and one was wounded, the officer said, speaking on condition of anonymity because he feared retribution. Hamza had defected two years ago to Prime Minister al-Maliki’s Dawa Party from the largest Shiite party, now known as the Supreme Islamic Iraqi Council.

Suspicion for the killing fell on Shiite extremists jockeying for power ahead of expected provincial elections.

Iskandariyah has a volatile mixed population that is about 60% Shiite and 40% Sunni. The Mahdi Army militia, loyal to radical Shiite cleric Moqtada al-Sadr, is active in the area, some 30 miles south of Baghdad. The Sadrists boycotted the previous provincial elections in January 2005, ceding most local leadership posts to Supreme Islamic Iraqi Council and other rival groups.

Rising internal violence threatens to plunge the southern Shiite heartland into political turmoil. The tensions pose a dilemma for the American military, which must decide whether to intervene in what essentially is a civil conflict even as critics step up calls to start withdrawing American troops.

Meanwhile, Sheik Muawiya Naji Jbara, the Sunni head of the Salahuddin Tribal Awakening Council, died from head injuries he suffered after a roadside bomb exploded as his convoy traveled near Samarra, 60 miles north of Baghdad, said his brother, Marwan Jbara. He said two guards also were wounded.