US defence officials said they suspected ground fire may have caused a UH-60 Black Hawk helicopter to crash Thursday in potato fields southeast of Fallujah, where guerrillas on January 2 shot down another US helicopter, killing an American soldier and injuring another.

"I heard a report that the pilot may have seen some sort of fire, and it may have hit a tail rotor," a senior defence official said, speaking on condition of anonymity. He said an investigation was underway.

Brigadier General Mark Kimmit, the US-led coalition's deputy operations chief in Iraq, said Thursday that the nine on board the helicopter were presumed to be US soldiers, taking to 225 the number of US combat fatalities since a declared end to major hostilities by US President George W. Bush on May 1.

Bush said he was "saddened" by the latest loss of life, White House spokesman Scott McClellan told reporters.

"Our men and women in the military are serving and sacrificing in Iraq for an important cause, a cause that is making the world a safer and better place," McClellan said.

In another incident involving an American aircraft Thursday, an Air Force C-5 was apparently hit by a missile as it took off, according to a senior US defence official.

"It looks like its number four engine was hit by a surface-to-air missile, but it was able to turn around, come back and land," the official said in Washington, speaking on condition of anonymity.

Late on Wednesday, one US soldier was killed and 33 wounded, along with one civilian, when guerrillas fired six mortar rounds into a military base close to Baghdad airport.

In another development early on Friday, insurgents fired two rocket-propelled grenades at a Baghdad hotel used by foreign companies, contracted by the US-led coalition, but there were no casualties, said security guards with the firms.

Meanwhile hundreds of US soldiers backed by air support sealed off much of toppled dictator Saddam Hussein's hometown of Tikrit early Friday, raiding houses and arresting a dozen suspected anti-coalition fighters.

And unrest continued in the ethnically-tense northern Iraqi city of Kirkuk late Thursday when a Kurd was shot dead by unidentified attackers and two bombs were defused, just days after deadly communal riots left four dead.

The latest attacks came as the coalition said it was beginning to release the first batch of several hundred Iraqi prisoners, freed in what was described as a goodwill gesture aimed at achieving reconciliation and curbing insurgency in post-war Iraq.

Two truckloads of around 60 Iraqi prisoners were released by the US-led coalition from Abu Gharib prison following a pledge by US overseer Paul Bremer to start freeing hundreds of detainees.

The prisoners were seen smiling and waving to relatives gathered outside the fortified prison, but it was impossible to confirm if they counted among the security detainees Bremer was releasing.

Bremer's spokesman Dan Senor told reporters: "I do know that 100 prisoners are ready to be released. I don't want to get into the business here of categorising every release, or what qualification they fall into."

Thursday's prisoner release was announced a day earlier alongside a 200,000-dollar reward programme aimed at reaping new intelligence.

The coalition is convinced the insurgency in Iraq is faltering despite the latest attacks, and is trying to capitalise on the demoralising blow of Saddam's arrest on December 13, even recruiting Saddam sympathisers who renounce their old leader.

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