An official from Heli Air, the Bulgarian company that owned the aircraft, said that besides three Bulgarian crew, there were six American passengers and two guards from the Philippines.

The six US citizens worked for Blackwater Security Consulting, which provides security for the US Embassy.

The attack, believed to be the first downing of a civilian aircraft in Iraq, came amid a surge in guerrilla violence and puts further pressure on leaders struggling to form a government nearly three months after elections.

The Mi-8 helicopter, which has both commercial and military use, was flying near the town of Tarmiya, about 40 kilometers north of Baghdad, when it was hit by ground fire, possibly by a rocket-propelled grenade, said US military officers.

The twin-engined aircraft can carry up to 24 passengers.

At the crash site - a deserted area - could be seen at least two charred bodies amid mangled and burning wreckage, including rotor blades and the two engines.

Insurgents frequently fire on US aircraft in Iraq and have brought down several helicopters.

A US Chinook transporter was shot down west of Baghdad in November 2003, killing at least 16 US soldiers troops and wounding more than 20.

Ten British troops died January 30 when a C-130 Hercules transport aircraft went down north of Baghdad. The cause remains unclear, but officials have said it may have been shot down.

The latest attack came amid a surge in guerrilla activity over the past two weeks, with more than 20 car bombings in Baghdad and an increase in ambushes and assassinations.

The violence threatens to eclipse efforts by elected leaders to form a government, amid growing tensions between Iraq's majority Shiite and once-dominant Sunni Muslim communities.

Hopes that a government would be announced Thursday were scotched when last-minute disagreements emerged between Shiites, who won the January 30 election, and other factions, including that of the interim prime minister, Iyad Allawi.

Iraqi President Jalal Talabani explained the new delay, telling Turkey's CNN Turk television: ``We want to see the Sunni Arabs represented as well.''

And ``negotiations also continue over the allocation of some posts,'' he added.

Disputes surfaced when Allawi rejected an offer to join the cabinet, said sources involved in the negotiations.

``The talks were going well, but the Shiites offered Allawi just two ministries, not the four that he wants, and he rejected the offer,'' said one insider of ministries offered to Allawi's political grouping.

``There was also continued disagreement over what ministries the Sunnis should get. The question really is whether the Shiites want to create a government of national unity or just a Shiite-Kurd government.''

The constant delays in forming a government have heightened tensions between Shiites and Sunnis at the leadership level and also appear to have fuelled the insurgency.

Shortly after the talks, Allawi narrowly escaped an assassination attempt, which was claimed by al-Qaeda in Iraq, the militant group led by Jordanian Abu Musab al-Zarqawi. The group has claimed responsibility for a series of attacks over the last month.

Immediately after January's election militant activity appeared to taper, with the country buoyed by the fact that more than eight million people had turned out to vote.

But in recent weeks, there has been a marked resurgence in violence.

Also Thursday, a roadside bomb hit a convoy carrying foreign security contractors on the road to Baghdad's airport, killing two people.

Three foreign contractors were killed on the same stretch of road Wednesday and two US soldiers in the same vicinity the day before.