BAGHDAD &#8212 An al-Qaida front group threatened to assassinate Sunni leaders who support American troops in Iraq as a Shiite bloc loyal to radical cleric Muqtada al-Sadr defected Saturday from the Iraqi government’s parliament base.

The two developments cast doubt over prospects for political and military progress in Iraq as the U.S. Senate gears up for a debate next week on Democratic demands for deeper and faster troop cuts than President Bush plans.

The threat against Sunni leaders came from the Islamic State of Iraq, which claimed responsibility for the assassination Thursday of Abdul-Sattar Abu Risha, the mastermind of the Sunni Arab revolt against al-Qaida in Anbar province. Bush met Abu Risha at a U.S. base in Anbar this month and praised his courage.

In a Web posting, the Islamic State said it had formed “special security committees” to track down and “assassinate the tribal figures, the traitors, who stained the reputations of the real tribes by submitting to the soldiers of the Crusade” and the Shiite-led government of Prime Minister Nouri al-Maliki.

In a statement, the purported head of the Islamic State, Abu Omar al-Baghdadi, said he was “honored to announce” a new Ramadan offensive in memory of Abu Musab al-Zarqawi, the founder of al-Qaida in Iraq killed last year in a U.S. air strike.

Hours after the announcement, a car bomb exploded late Saturday in a mostly Shiite area of southwest Baghdad, killing at least 11 people lined up to buy bread at a bakery. Two of the dead were children, police said.

“We rushed outside the house after hearing the sound of the explosion. I could see the bakery and a nearby pickle shop on fire,” said Abu Ahmed, a 36-year-old Shiite government employee. “The wounded were screaming for help as the ambulances were arriving.”

The blast occurred at the start of iftar, the evening meal at which Muslims break their dawn-to-dusk Ramadan fast. The bloodshed was a blow to government hopes that a peaceful Ramadan would demonstrate the success of the seven-month operation in the capital.

Also Saturday, the U.S. military said a soldier from the Army’s Task Force Marne was killed and four were wounded the day before when a bomb exploded near their foot patrol.

The Sunni revolt which Abu Risha spearheaded has led to a dramatic improvement in security in Anbar, although the province remains unstable. Nevertheless, the decline in violence in Ramadi and other Anbar cities has been one of the major success stories for the U.S. mission in Iraq.

A prominent Sunni sheik told The Associated Press that the province’s leaders would not be intimidated by al-Qaida threats and would continue efforts to drive the terror movement from their communities.

“We as tribesmen will act against the al-Qaida, and those standing behind it who do not want us to put an end to it,” Ali Hatem al-Suleiman said.

Still, the al-Qaida threats and the assassination of Abu Risha, one of the best protected tribal figures in Iraq, could cause some tribal leaders in other Sunni provinces to reconsider plans to stand up against the terror movement.