The attack occurred at 10:00 am (0600 GMT) when assailants fired an RPG at a US Humvee light multi-wheeled vehicle near a gas station in the al-Mustansiriya neighborhood, they said.

Four US troops were killed and two others wounded, they said. The casualties were immediately removed from the scene.

Witnesses said the Humvee was totally burnt.

An Iraqi civilian was also wounded and taken to hospital, said the witnesses, confirming that his 18-seat transport bus parked by the gas station was completely torched.

US troops immediately cordoned off the area, preventing journalists from approaching the scene.

A US military official told AFP that there was still no word on US casualties and that "the incident has not been qualified as an attack yet."

Increasingly regular attacks have targeted US-led coalition forces occupying Iraq, with at least 21 US and six British soldiers killed since May 1, when US President George W. Bush declared the war launched to oust Saddam Hussein's regime over.

WAR.WIRE