Washington: US President George W. Bush has refused to criticise a US security company in Iraq accused in a shooting that left 11 civilians dead.

Bush on Thursday said investigators first need to determine if the security guards of Blackwater USA violated rules governing their operations.

"Obviously, to the extent innocent life was lost, you know, I'm saddened," Bush said at a press conference.

Iraqi Prime Minister Nouri Al Maliki has urged the US Embassy to find another security firm, saying he cannot tolerate “the killing of our citizens in cold blood”.

Blackwater officials said its guards were just responding to an armed attack on Sunday against a US convoy. Iraq officials said the guards fired indiscriminately at civilians.

A US-Iraqi commission is looking into the shooting.