A US soldier was killed and near the hotspot of Falluja, witnesses said, and a US MH-53 helicopter crashed southeast of here yesterday and marines who rushed to secure the crash site came under attack and sustained casualties, a marine officer said.

The officer said it was not known how many people were aboard the helicopter, which can carry up to 55 people.

He said the aircraft did not belong to the Marine Corps, but to another US government agency. Around 40 foreign hostages from 12 countries are currently being held by insurgents, and the FBI is involved investigations into the abductions, a coalition spokesman said.

Foreign nationals are being urged to flee Iraq as governments and private companies react to growing insecurity and a wave of kidnappings.

And Russia's biggest contractor in Iraq, Tekhpromexport, is pulling its 370 staff out of Iraq because of the spate of abductions of foreigners.

"The eight hostages have been freed and are now at home," Interenergoservis general director Alexander Abramov told Interfax. There was no immediate word on the identity of the kidnappers or what had led to their swift release.

Seven Chinese nationals seized separately near Falluja were freed on Monday. Three Czech journalists were reported missing.

The fate of three Japanese hostages whose captors threatened on Thursday to kill them unless Tokyo withdrew its troops from Iraq remained unclear.

Fighting erupted in al-Karma, near Falluja, and a Reuters photographer at the scene said he saw clouds of smoke after US helicopters fired missiles at targets in the Sunni town.

The US army said a bomb attack on a convoy killed a US soldier yesterday and wounded another soldier and a civilian contractor. The convoy, travelling from Baquba to Najaf, was hit just after midnight by a roadside bomb planted south of Baghdad.

Meanwhile, an aide to radical Shia cleric Moqtada al-Sadr has been released from US military custody after five hours in detention, witnesses say.