WASHINGTON - THE US government on Tuesday pleaded for patience from Baghdad as a private US security firm's role in a deadly gun battle tested US claims that war-torn Iraq is a sovereign nation.

The White House, the Pentagon, and the US State Department were grappling with how to curb the damage from Sunday's clash in which Blackwater contractors apparently killed civilians, fueling anti-US sentiments in Iraq.

A top Iraqi judge has said Blackwater could face trial over the incident, in which some of its guards, who were escorting US embassy officials, opened fire in a Baghdad neighbourhood, killing 10 and wounding 13.

But Blackwater denies any wrongdoing, and US legal experts said the contractors may be immune from prosecution under a measure conceived by the US-led Coalition Provisional Authority not long after the March 2003 invasion.

And, US officials said, it was unclear whether - or if - any US nationals involved would be tried under US or Iraqi law over the incident, which Iraqi Prime Minister Nouri al-Maliki has angrily branded a 'criminal' act.

'That bit of it will come at the very end' of a probe into what happened, State Department spokesman Sean McCormack said one day after US Secretary of State Condoleezza Rice telephone Mr Maliki to ease his concerns.

'You would have to have a precise set of facts in order to be able to determine the various applicable legal authorities and whether or not ... there were any laws that were broken,' said Mr McCormack.

'We want to be as open and transparent and cooperative as we possibly can with the Iraqis. I think that this is going to be a process that unfolds over a period of time. I can't tell you how long,' he said.

The State Department and Iraq's government were conducting parallel probes into the incident, but 'look for those investigations to merge and to have a joint investigation pretty soon,' a US official said on condition of anonymity. -- AFP