The incident occurred on July 14 when Bayez Ismail, a member of the ruling Kurdistan Democratic Party, and his brother Ari, were travelling in a private car that came too close to a convoy escorted by a private security detail.

The guards opened fire on them. Ari sustained a serious head injury.

"We have asked the US army to hand over the people who fired on the car," said Arbil police chief, Colonel Farhad Salim, adding he had proof they were US nationals.

"This isn't the first time such an incident happens," he said, claiming that in the past US forces had paid out financial compensation in such cases.

The US embassy said in a statement: "A joint US-Iraqi (Kurdish) Police preliminary investigation has been conducted to determine the facts of the case. The results of that investigation are in dispute."

Private security guards, most of them former soldiers, were granted immunity from prosecution by Iraq's former US administrator Paul Bremer in June 2004 and their combined forces number more than 20,000, the second largest foreign force in Iraq after the US army.

The US embassy said that as a result of disagreements between Kurds and US officials in the case, "armed uniformed local troops have begun to stop PSD (personal security details) in Arbil in an effort to identify the persons whom they believe may have been the shooters of the civilian vehicle.