An Iraqi soldier has shot dead at least two US soldiers and wounded six others in the northern Iraqi city of Mosul.
The incident happened at an Iraqi military base in the Zanjili district of the city, the US military said, not on a joint patrol as earlier reported.
The Iraqi trooper turned his gun on patrol members, the US military said, before being shot dead himself.
Reports suggested the man had been involved in an altercation with a US soldier before opening fire.
The US military said it was still investigating the incident.
There have been other incidents of Iraqi troopers attacking the Americans they are working with but this, reports the BBC's Andrew North in Baghdad, would be one of the most serious.
Thousands of US and Iraqi troops usually work together across the country every day without serious problems, our correspondent adds, although cultural and language barriers can be a source of misunderstanding.
In Baghdad, meanwhile, at least 21 people were killed and more than 85 injured in a series of bombings - the third day in a row such attacks have taken place.
Overall security in Iraq has improved significantly in the past year but recent weeks have seen a rash of incidents that have undermined public confidence.