A US miltary officer said the attack happened in Saddam Hussein's hometown of Tikrit on Wednesday.

He said rocket-propelled grenades were fired at the convoy in the centre of Tikrit, 170 km north of Baghdad.

However, the officer did not give any further details.

The US military is on high alert after Tuesday's deadly bomb attack, which devastated a UN compound in Baghdad.

There are about 230,000 US troops serving in and around Iraq.

At least 122 American soldiers have now been killed in the country since the White House declared major combat operations in Iraq over on 1 May.