U.S. military helicopter crashed Saturday during a "combat air patrol" southwest of Baghdad, but the status of the crew was unknown, according to the American command.

Meanwhile, pressure mounted on Prime Minister Ibrahim al-Jaafari to step aside as the Shiite bloc's nominee for a second term, with some fellow Shiites urging him to withdraw to break the deadlock over a new government amid increasing sectarian violence.

A U.S. statement said the helicopter went down about 5:30 p.m. during a combat patrol southwest of the capital but gave no further details, except to say that the fate of the crew was unknown. The statement did not identify the type of helicopter.

It was the first loss of a U.S. helicopter since three of them crashed in a 10-day period in January, killing a total of 18 American military personnel. At least two of the helicopters were shot down.

The U.S. command also said a Marine was killed Friday during combat operations in Anbar province west of the capital. The Marine's death brought to at least 2,328 the number of members of the U.S. military who have died since the Iraq war started in March 2003, according to an Associated Press count.

The violence came as U.S. officials expressed increasing impatience with the slow pace of government talks following the Dec. 15 elections.

U.S. Ambassador Zalmay Khalilzad urged the Iraqis to speed up the process to prevent the country from sliding into civil war.

"The terrorists are seeking to provoke sectarian war, and Iraq needs a government of national unity in the face of this threat," Khalilzad said in a statement released Saturday.

"This government needs to have a good program to govern from the center, and needs good ministers who are competent. Iraq is bleeding while they are moving at a very slow pace," he added.

At least 22 people were killed Saturday in fresh violence in Baghdad and Basra, Iraq's two largest cities. Six others — all Shiite men — died Friday evening when gunmen opened fire on a minibus near Balad Ruz, 45 miles northeast of Baghdad, the town's mayor, Mohammed Maarouf, said.

U.S. officials believe formation of a government of national unity would be a major step toward calming the insurgency and restoring order three years after the U.S.-led invasion toppled Saddam Hussein. That would enable the U.S. and its coalition partners to begin withdrawing troops.

But talks among Iraqi political leaders have bogged down, prompting Sunni Arab and Kurdish politicians to call for al-Jaafari's replacement. The Shiites get first crack at the prime minister's job because they are the largest bloc in parliament.

On Saturday, a former Shiite Cabinet minister, Qassim Dawoud, joined those calls, saying al-Jaafari should step aside to break the deadlock.

"I personally asked that he withdraw his nomination," Dawoud told The Associated Press.

Dawoud later said four major parties within the Shiite alliance had agreed to "reconsider" al-Jaafari's nomination. But Jawad al-Maliki, a member of the prime minister's Dawa party, insisted to Al-Arabiya television that the alliance "is united in its position" and "is backing its candidate," meaning al-Jaafari.

Other Shiite officials, speaking on condition of anonymity because of the sensitivity of the issue, said Dawoud was not alone in his opposition to al-Jaafari, and that representatives of major factions within the Shiite alliance would decide soon whether to withdraw the nomination.

Al-Jaafari, a physician who spent years in exile in Iran and Britain, edged out Vice President Adil Abdul-Mahdi for the nomination during an alliance caucus in February thanks to the support of radical Shiite cleric Muqtada al-Sadr.

The prospect of a prime minister politically beholden to the vehemently anti-American al-Sadr has alarmed both Iraqi and U.S. officials.

Al-Sadr's bloc in parliament reaffirmed its support for al-Jaafari.

"We will not abandon our decision regarding al-Jaafari's candidacy," the bloc's leader in parliament, Salam al-Maliki, told Al-Arabiya television on Saturday.

Officials said that despite opposition to al-Jaafari, Shiite leaders were hesitant to move against him for fear of splitting the alliance.

Tension between the rival Shiite and Sunni Muslim communities escalated following the Feb. 22 bombing of a Shiite shrine in Samarra and reprisal attacks on Sunni mosques and clerics in Baghdad, Basra and other religiously mixed cities.

On Saturday, gunmen killed three ice cream vendors in Baghdad's southern neighborhood of Dora, while a butcher and his son were killed and another son was wounded in east Baghdad, police said.

The owner of an air conditioner repair shop was shot to death on his way to work in western Baghdad.

Police also found nine bodies, mostly young men who were shot in the head or strangled in Baghdad. Witnesses also told police they saw three gunmen in a BMW pull a handcuffed man out of the car and shoot him near a highway in west Baghdad.

In Basra, a Sunni sheik was killed by gunmen in a speeding car when he left his home in the southern city. Two policemen also were killed in a bombing south of Basra, police said.

West of Baghdad, U.S. and Iraqi troops killed three suspected insurgents, including a woman, and captured three others Saturday in an operation in Amiriyah in Anbar province, the U.S. military said.