A US helicopter has crashed north of Baghdad, with witnesses quoted by Reuters news agency saying it was shot down by a rocket.

One witness said on Monday US troops surrounded the crash site and that smoke was rising from the area near the small town of Mishahda.

The US military confirmed the crash and said there was no immediate news of its two-man crew.

"A task force Ironside helicopter did go down this morning at 8.20am (0520 GMT), a spokesman said.

The cause of the accident, a few kilometres north of the capital, is under investigation, he added.

Two US pilots were killed on Friday when their reconnaissance helicopter was apparently shot down in northern Iraq, while eight US soldiers and four civilians died a week earlier when their Black Hawk helicopter crashed near the town of Tal Afar in northwestern Iraq.

Elsewhere, a bomb exploded near a convoy of civilian vehicles in Baghdad on Monday, triggering a shoot-out between Iraqi police and fighters, but nobody was hurt in the violence, a police officer said.

The explosion was the first in the city centre after a week-long Muslim holiday lull.

Four four-wheel drive cars, frequently used by foreign security contractors and diplomats to move around Baghdad, were targeted in the attack, the police officer on the scene told AFP, speaking on condition of anonymity.

It was not immediately clear who had been travelling in the convoy, which appeared to have been escorted by two overhead helicopters.

"After the explosion, insurgents in a nearby building started shooting at the convoy," the police officer said.

"Iraqi police returned fire. We then stormed the building but were unable to find any of the culprits."

The bomb only caused minimal damage to one of the vehicles in the convoy, the officer said.

Earlier, an Interior Ministry official said US and Iraqi forces had been involved in the shooting. A US military spokesman said he was looking into the report of the explosion.

In other violence, a policeman was killed in the north of the capital and one civilian and another policemen were wounded in a roadside bomb explosion in the west of Baghdad.

On Sunday, police found the bodies of seven unidentified men shot to death in the capital.

PHOTO CAPTION

The crumpled wreckage of a U.S. military OH-58D Kiowa helicopter lies on its side, Friday, Jan. 13, 2006, in Mosul, 360 kilometers (225 miles) northwest of Baghdad, Iraq. (AP)