"Four coalition forces members were killed when two aircraft went down in northern Iraq at approximately 2:15 am (2315 GMT Sunday)," a US army spokesman said in an initial statement.

The cause of the incident is unknown but does not appear to be the result of "enemy action," a separate US military statement said later.

An Iraqi military official told AFP two helicopters were involved in the incident, while police said the crash occurred near the northern oil city of Kirkuk.

However, the Nakshabandiya insurgent group -- close to executed president Saddam Hussein's still fugitive deputy Izzat Ibrahim al-Duri -- later said that it had shot down the coalition aircraft.

Asked about the claim, the US military issued a further statement, which said: "There is no indication that the helicopter crash is a result of enemy action."

The Nakshabandiya handed out leaflets on the streets of Kirkuk, saying that they had shot down two helicopters and "would soon show a video," an AFP correspondent witnessed.

A statement on their website said: "We announce with pleasure the shooting down of two helicopters of the American enemy. It was a night ambush from the Anti-Aircraft Resistance Brigade. It was two Blackhawks shot down Sunday evening at 10.30pm in Hawijah," 50 kilometres (30 miles) west of Kirkuk.

The group said that the attack had killed "more than 20" soldiers and that the video would show the "entire operation".

Hawijah is a largely Sunni Arab town within the disputed oil province of Kirkuk and was the scene of a massive US operation to try to capture Ibrahim in late 2003.

The US military is currently taking a back seat to an increasingly large Iraqi force made up of 560,000 policemen and 260,000 military personnel, with the US providing logistic and air support on request.

According to the Pentagon, 143,000 American troops are deployed in Iraq.

Under an agreement signed between Washington and Baghdad in November, the US military is due to withdraw its combat troops from Iraq by the end of 2011 and to pull them back from built-up areas by the end of June this year.

At least 4,236 US military personnel have died in Iraq since the 2003 invasion, according to an AFP tally based on the independent website www.icasualties.org, including the deaths on Monday. Fifteen troops have died so far this year.

A US OH-58 Kiowa helicopter crashed on November 15 last year in the main northern city of Mosul, the last such reported incident.

A month earlier, three US soldiers were wounded when two UH-60 Blackhawk helicopters crashed during landing at their base in Baghdad.

The UH-60 is the workhorse of the US military and operates in pairs. Each aircraft crew usually comprises two pilots and two gunners. Blackhawks routinely throw magnesium flares to deflect any heat-seeking missiles fired at them.

The deadliest single crash was on November 15, 2003, when two Blackhawks collided near Mosul, killing 17 soldiers.

In August 2007, a Blackhawk transport chopper crashed in northern Iraq, killing 14 US military personnel.

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