A US military spokesman confirmed the explosion on Wednesday. The blast occurred in front of an office used by US personnel on Tuesday.
The cause of the explosion was not immediately clear.
US-led forces have come under attack almost every day since they toppled Iraqi President Saddam Hussein five months ago.
But until now US personnel have been able to regard the Kurdish-controlled northern areas of Iraq as relatively safe.
The party in charge of the area, the Kurdish Democratic Party of Massud Barzani, has been seen as a friendly force and is unlikely to be suspected of being behind the attack.
A few hours earlier, Egypt's official MENA news agency said large quantities of explosives had been set off near a KDP checkpoint.
Barzani's KDP have controlled most of Irbil since the early 1990s.