Efforts to lure the United Nations into a renewed role in the planned transfer of power from the US-led coalition to Iraqis have been boosted by a call from the country's top Shiite cleric to halt protests against the plans.

But the US casualty list continued to mount after two pilots were killed Friday when their military reconnaissance helicopter went down in northern Iraq. The cause of the crash was not immediately clear.

The arrival of the UN liaison team in Baghdad has ended a three-month absence by UN international staff since a spate of deadly attacks last year.

The team was charged only with opening up channels of communication with the coalition, a spokesman said.

A separate security assessment would be needed if UN chief Kofi Annan announces the dispatch of a mission to assess the viability of immediate elections, as he is widely speculated to do next week.

But top Shiite leader, Grand Ayatollah Ali al-Sistani, ordered his followers to give the world body space to set up such a mission, a week after threatening a campaign of civil disobedience unless the coalition agrees to organise elections for the first post-occupation government, due to take power by the end of June.

It is vital "to wait until the United States and the UN clarify their positions on the election procedure to choose the nature of the next Iraqi government," Sistani's spokesman Sheikh Abdul Mahdi al-Karbalai said in his sermon at the main weekly prayers in the Shiite pilgrimage city of Karbala.

Tens of thousands of Shiites have taken to the streets over the past week to back Sistani's demands, prompting Washington to seek UN support as it tries to shore up its plans for a swift handover of sovereignty without prior polls.

While a UN evaluation mission would address the demands of Sistani and the mainstream Shiite hierarchy, it stirred the wrath of Shiite radical leader Moqtada Sadr, who branded the world body "dishonest" and subservient to America.

Nevertheless, Ahmed Chalabi, a Pentagon-backed member of the Governing Council said it would be "possible" to hold Iraqi elections before the scheduled power transfer, in comments made Friday to US think-tank, the American Enterprise Institute.

Meanwhile, US officials in Washington revealed that two senior al-Qaeda suspects had been captured in Iraq.

"These are significant developments," said a US official, who asked not to be identified.

Hasan Guhl, a Pakistani was captured Thursday where he was believed to be scoping out the turf for organizing Al-Qaeda operations in Iraq and working with like minded Islamic extremists.

"He is a very significant player," the official said. "He's a longtime facilitator of al-Qaeda operations in terms of moving both people and money. He has an extensive network of contacts all over the world."

In another major break, US forces captured Husan al-Yemeni, the leader of an Ansar al-Islam cell in the flashpoint town of Fallujah, on January 15, said another US official, who also asked not to be identified.

"He is the most senior Ansar al-Islam person that's been caught to date," said the official.

But there has been no let-up in the nine-month insurgency against the US occupation in mainly Sunni areas north and west of Baghdad.