Nine foreign troops have been killed in a helicopter crash in northwestern Afghanistan, the International Security Assistance Force (Isaf) has said.

US defence officials told Reuters that all of the Nato troops killed in Tuesday's crash were Americans.

Four others were also injured, including two Nato troops, an Afghan soldier and a US civilian.

The Taliban claimed to have shot down the helicopter, but Nato said there were "no reports of enemy fire in the area".

Sue Turton, Al Jazeera's correspondent reporting from the Afghan capital, said the Taliban sent out a statement saying they had killed 16 foreigners in the incident.

"[Isaf are denying the helicopter was attacked] and police on the ground have told us they can't see evidence that the helicopter was brought down," she said.

"[However] we have spoken to an eye witness in the area who said he saw a rocket hit it."

The injured had been taken to an Isaf medical facility, the statement said. It gave no details on the nationalities of those killed. 

The crash occurred in northwestern Zabul province, according to a Nato official, who spoke on condition of anonymity because he was not authorised to disclose the location of the crash. Mohammad Jan Rasoolyar, a spokesman for the provincial governor in Zabul, said the helicopter went down in Daychopan district.

At least 529 foreign troops have been killed so far this year in Afghanistan.

The toll makes 2010 the deadliest year since the US-led invasion in 2001, according to the monitoring website iCasualties.org.