Seventeen soldiers have been killed and five others wounded in the crash of two US military helicopters in the northern Iraqi city of Mosul. The casualty toll makes this the deadliest single strike on US forces since the war began. One soldier remains unaccounted for, the US military said. An occupation force spokesman has confirmed that two UH-60 Black Hawk helicopters had come down. There was no word on possible Iraqi casualties on the ground. One of the helicopters that crashed on Saturday was hit by a rocket propelled grenade (RPG), a US officer at the scene said. "I know one of the helicopters was hit by an RPG on the tail wing," said the officer who declined to be identified. An Iraqi police officer said that he saw assailants ambushing a US foot patrol in the area, prompting the intervention of a Black Hawk helicopter. A missile was then fired towards the chopper, which crashed into a second Black Hawk as it tried to dodge the missile, he said. **Area Sealed off*** American soldiers have sealed off the crash site in a residential neighbourhood in Mosul. A rapid reaction force was securing the area and investigating. Saturday's double crash brings to five the number of US helicopters downed in Iraq since the beginning of November. On 2 November, a US Chinook helicopter was shot down west of Baghdad, killing 16 soldiers. Just five days later, a Black Hawk was shot down in Saddam Hussein's hometown of Tikrit, killing all six people aboard. Another Black Hawk was forced down near Tikrit, but there were no casualties. **PHOTO CAPTION*** The Black Hawk is the US army's frontline utility helicopter, designed to carry 11 combat-ready troops. It is also used for medical evacuations.