For the third time in as many days, Washington offered high-level assurances to Moscow that it took the matter extremely seriously and that a full investigation was underway.

"This is a matter of extreme seriousness," deputy State Department spokesman Philip Reeker said. "The sanctity of diplomatic personages is very important to us as well as the international community and must be upheld."

"No determination has yet been made as to precisely what happened or whose forces were involved -- we are trying to establish these facts," he said in a statement, terming the Sunday incident "regrettable."

"We are in close communication with the Russian government on this matter," Reeker said. "We will communicate fully the findings to the government of Russia as soon as they are available."

The statement was released after similar assurances were given to Russian President Vladimir Putin by US national security advisor Condoleezza Rice on Monday in Moscow and to Russian Foreign Minister Igor Ivanov by Secretary of State Colin Powell in a Sunday telephone call.

The statement also followed allegations by Russia's ambassador to Iraq, who was slightly wounded in the incident along with four others, that US forces had deliberately opened fire on the convoy as it left Baghdad.

According to eyewitness accounts, the convoy was caught in crossfire between US and Iraqi soldiers although the reports were not clear as to which side shot first.

The Russian envoy, Vladimir Titorenko, told the ITAR-TASS news agency on Monday that the convoy had not had not deviated from the agreed-upon time and route for departure and that US forces had deliberately shot at the vehicles.

Titorenko's account directly contradicted preliminary US reports from the field that Iraq may have set up the attack by instructing the convoy's drivers to take a different route than had been agreed upon with US diplomats.

A senior US official told AFP on Monday there were indications that the Iraqis had tried to create an international incident by altering the route of the convoy to pass through a contested area west of Baghdad.

On Tuesday, the same official maintained that the convoy had taken a different route, but said it was possible that the Iraqis might have been trying to set a trap for US troops that the Russians may have stumbled into.

"We believe he took the wrong road," the official said, referring to Titorenko. "He was told to take one road, he took another.

"It's a war zone and things like this can happen, but if our guys were involved, they certainly weren't trying to do any damage to the convoy or the people in it," the official said on condition of anonymity.

The incident has cast a new pall over already strained ties between the United States and Russia, which is fiercely opposed to the war in Iraq.

Even before the convoy attack, Russia had complained that the US-led bombing of Baghdad was endangering its diplomats stationed there.

US officials have also accused Russian firms of illegally selling weapons and military technology to Iraq in violation of UN sanctions and complained that Moscow has done nothing to stop such transfers, a charge that Russia denies.

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