A US army helicopter has crashed near the city of Mosul, killing two pilots on board, after coming under fire from fighters on the ground, US officials and witnesses say.

Witnesses reported seeing men armed with heavy machine guns open fire on the OH-58D Kiowa Warrior, a two-seat, single-engine helicopter, in al-Sukar district, north of Mosul, about 390km north of Baghdad.

The US military said on Friday: "Two pilots were killed on Friday when their OH-58D Kiowa helicopter went down in the city of Mosul... The aircrew members were recovered from the aircraft."

Lieutenant-General John Vines, the deputy commander of the US-led force in Iraq, confirmed that "the indicators are that it was due to hostile fire".

A Reuters cameraman in al-Sukar said US forces swiftly sealed off the area after the crash. The helicopter was completely destroyed and wreckage was strewn about.

The US military said the aircraft had been conducting patrols with another OH-58D when it crashed.

Friday's was the second US helicopter crash in Iraq in less than a week. A military UH-60 Black Hawk helicopter crashed on Saturday, killing all 12 on board.

The US military believes the Black Hawk may have been brought down by bad weather, but the cause is still under investigation.

Saddam trial chief judge to quit

Rizgar Amin, the chief judge in the trial of Saddam Hussein, plans to step down, a source close to the judge has said.

"He wants to withdraw," the source said on Friday. "He will oversee the next sitting and then announce his reasons for withdrawing."

Amin's next hearing is on 24 January.

Asked why the Kurdish judge, based in the northern city of Sulaimaniya, wanted to pull out of a trial that has made his face familiar around the world during long days of television coverage, he would say only: "It is too difficult."

The killing of two defence lawyers has already highlighted problems with the process in a country mired in a virtual civil war that pits Saddam's fellow minority Sunni Arabs against a US-backed government run by Shia Muslims and ethnic Kurds intent on hanging a man they say massacred their peoples.

Kidnapping and murder have become commonplace and human rights groups have questioned the wisdom of pushing ahead with a trial in Baghdad rather than an international process in The Hague or elsewhere.

Precedent

Court officials were not immediately available for comment.

There is already a precedent in the trial, which opened on 19 October, for replacing one of the panel of five judges, so in principle Amin's departure may cause little upset.

But in practice, the resignation of the most visible face of the court outside of the dock may be an embarrassment for the Iraqi government and US officials keen to show the world that Iraqis are capable of giving their former leader a fair trial.

Amin, 48, told Reuters in November that his family worried about him and he had taken on two bodyguards after pressure from friends. But he stressed: "A judge should never be afraid because he defends justice and the law."

Hidden faces

Only one other of the five judges on the panel has allowed himself to be seen on camera and many of the witnesses called to testify have spoken behind a screen with their voices distorted to avoid retribution.

After several hearings of crimes against humanity committed against over 140 Shia men from the town of Dujail, Amin was criticised by some observers for allowing Saddam to speak at length, making allegations, including of his maltreatment at American hands.

The judge, whose dry wit and courteous manner have been features of the trial since its first day, has rejected the criticism and insisted the defence should have a fair hearing.

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 northwest of Baghdad, Iraq. (AP)