An Al Qaeda front group threatened to assassinate Sunni leaders who "stained the reputations" of their people by supporting the Americans as the Iraqi government's parliament base fragmented Saturday with the defection of a hardline Shiite bloc.
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 Qaeda 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.
"We will publish lists of names of the tribal figures to scandalize them in front of our blessed tribes," the statement added.
In a second 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 Qaeda 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 Qaeda threats and would continue efforts to drive the terror movement from their communities.
"We as tribesmen will act against the Al Qaeda, and those standing behind it who do not want us to put an end to it," Ali Hatem al-Suleiman said.
Still, the Al Qaeda 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.
With U.S. and Iraqi overtures to the Sunnis under threat, the government faced a deepening political crisis with the announcement that followers of the radical Shiite cleric Muqtada al-Sadr were withdrawing from the Shiite alliance in parliament. Al-Sadr's followers hold 30 of the 275 parliament seats.
The announcement, made to reporters in Najaf, means the Shiite-led government can count on the support of only 108 parliament members -- 30 short of a majority. However, it could probably win the backing of the 30 independent Shiite parliamentarians, as well as some minor parties.
Still, the decision by al-Sadr's followers will complicate further U.S.-backed efforts to win parliamentary approval of power-sharing legislation, including the oil bill and an easing of curbs that prevent former Saddam Hussein supporters from holding government jobs.
Al-Sadr's decision will also sharpen the power struggle among armed Shiite groups in the south, which includes major Shiite religious shrines and much of the country's vast oil resources.
The Sadrists had been threatening to bolt the Shiite alliance for several days. But tensions rose after arrest warrants were issued against Sadrist officials in the holy city of Karbala in connection with last month's Shiite factional fighting there.
The warrants, which were made public Saturday, angered the Sadrists, who said the government was provoking them despite recent gestures by al-Sadr, including a six-month halt to military operations by his Mahdi Army militia.
In the northern city of Mosul, authorities ordered all vehicles off the streets from late Saturday until sunrise Monday to enable security forces to search for explosives.
The ban was ordered after the Iraqi military announced it had found six booby-trapped cars and a would-be homicide bomber in the city Saturday.