An Iraqi soldier opened fire on US troops during a joint patrol in the northern city of Mosul on December 26, killing two and wounding three others along with a civilian interpreter, Iraqi and US officials said on Saturday.

US military spokesman Lieutenant-Colonel James Hutton said it was not clear why the Iraqi soldier had opened fire, but two Iraqi generals told Reuters the attacker had links to Sunni Arab insurgent groups and had been detained. “The incident occurred as US and Iraqi army soldiers were conducting operations to establish a combat outpost,” he said.

It is believed to be the first reported incident in which an Iraqi soldier has deliberately killed US servicemen since the US-led invasion in 2003. In response, the Iraqi army has tightened screening of new recruits in the 2nd Division, which controls the Mosul region, and is carrying out more thorough background checks on serving soldiers.US and Iraqi troops have been conducting patrols as part of a new US counter-insurgency strategy to curb violence.