BAGHDAD - Leaders of Iraq's ruling Shi'ite Alliance bloc called publicly for the first time on Saturday for Ibrahim al-Jaafari to step down as prime minister to break weeks of deadlock over a national unity government.

The call declared by one leader and echoed, anonymously, by others sparked consternation in the Alliance ranks as parties held their latest round of talks on a grand coalition with Kurds and Sunnis, who are adamant in their rejection of Jaafari.

Officials say a unity government, more than three months after December's election, is vital to averting civil war after five weeks of spiraling sectarian bloodshed.

"I call on Jaafari to take a courageous step and set a fine example by stepping down," Kasim Daoud, a senior member of the independent group within the Alliance, told Reuters.

A top aide to Jaafari immediately rejected the call. Jawad al-Maliki told Reuters Jaafari would go on "until the end."

But other senior Alliance officials, speaking anonymously, confirmed four of seven main groups within the bloc wanted Jaafari to give up the nomination for a second term if, as is all but certain, he fails to persuade minority Sunni and Kurdish parties to drop their refusal to serve in a cabinet under him.

"Daoud's call is supported by at least 60 percent of Alliance members of parliament," another senior Alliance official from another group within the bloc told Reuters.

"We need another 24 hours before starting the battle" to pressure Jaafari into resigning, he added.

The United States has stepped up pressure on Iraqi leaders to form a coalition of Shi'ites, Sunni Arabs and Kurds, seen as critical to putting a lid on sectarian violence that has killed hundreds since a major Shi'ite shrine was bombed a month ago.

Privately, rival Alliance leaders have been turning against Jaafari but the call on Saturday was their first public demand.

SUPPORT WANING

Shi'ite leaders have also said Washington has told them it does not want Jaafari -- criticized by Sunnis and Kurds for failing to stem the violence -- to continue as prime minister.

"There is a broad trend inside the Alliance who want Jaafari to do this (step aside) and we expect him to do so," Daoud said.

"We have stood behind him for 50 days and today we have reached the conclusion that there should be a prime minister for all Iraqis, not just one group."

Alliance officials said the seven key groups inside the bloc had met on Thursday and Friday and concluded by a four to three majority to give Jaafari just days to persuade the Kurds, Sunnis and secular leaders to drop their opposition to him.

That seems highly improbable but a committee of three Alliance officials was meeting on Saturday with Kurds and Sunnis.

The minority groups had formally written this week to Alliance leader Abdul Aziz al-Hakim urging him to produce a more acceptable candidate. Jaafari beat a candidate from Hakim's SCIRI party by a single vote in an internal ballot in February.

As by far the biggest bloc in parliament, with a near- majority of 128 seats in the 275-seat chamber, the Alliance has the right to nominate who will fill the most powerful job.

One alternative may be the defeated SCIRI candidate, Vice President Adel Abdul Mahdi. However, his role in that ballot may count against him and leaders may prefer a compromise third man.

Among other names mentioned are Finance Minister Ali Allawi, deputy parliamentary speaker Hussain al-Shahristani, Fadhila party leader Nadim al-Jaberi and Qasim Daoud himself.

Officials said Jaafari had the support of his own Dawa party, its Dawa-Iraq allies and the movement of Iranian-backed cleric and militia leader Moqtada al-Sadr. SCIRI and its Badr allies, the independents and Fadhila were against Jaafari.

U.S. VIEW

U.S. diplomats deny accounts from SCIRI and other Alliance officials that Washington has pressured Hakim to drop Jaafari.

However, a U.S. diplomat said on Saturday it was Washington's analysis that any prime minister must be both competent and able to unite Iraqis -- and that Jaafari did not score well on those criteria. The United States, however, had no preferred candidate in mind and would not impose its views.

While Shi'ite-Sunni violence has been rising since the mosque attack -- dozens of often badly mutilated corpses turning up every day on the streets of Baghdad alone -- monthly U.S. troop casualties are at about a two-year low.

There were at least 29 U.S. military deaths in March, the Pentagon said on Friday, the lowest monthly toll since 20 in February 2004, which was the lowest of the three-year war.

It was the fifth straight fall in the monthly toll.

There have been 2,327 U.S. military deaths in the war.

Analysts say the reasons for the fall are twin: a more defensive posture by the military and violence being redirected away from the U.S. to the sectarian conflict.

On Friday, five civilians were shot dead by unknown gunmen near the city of Baquba, 65 km (40 miles) north of Baghdad.