The US forces killed six Iraqi police and seven militiamen after coming under fire during a raid in central Baghdad to arrest a police lieutenant accused of running a terror cell, the military said.

An American unit called in an air strike to protect themselves after coming under "heavy and accurate fire" from an Iraqi police checkpoint during the pre-dawn operation.

The strike was sought in order to "prevent further escalation of fire between the Iraqi police checkpoint and coalition forces," said a US military statement.

"The close air support was directed in front of the Iraqi police, not at them to prevent further casualties," it added.

The detained "terrorist" led a unit of the Special Groups, the military's name for Iraqi militant cells supported by the Iranian Revolutionary Guards' covert Qods Force, the statement said.

The US military regularly charges the Qods Force of arming and funding Iraqi extremists to attack troops in Iraq. Tehran denies the charges.

US forces work alongside Iraqi police to train them and prepare them to take over security responsibility in Iraq, but often find their nominal allies are infiltrated by militant and insurgent sympathisers.

In a separate statement the military said its forces had also killed two suspected militants and detained 19 more in raids carried out in Baghdad and western Iraq.

Meanwhile, an Iraqi journalist working with The New York Times in Baghdad was shot dead on Friday, the newspaper's Baghdad bureau chief said.