Nine foreign troops were killed in a NATO helicopter crash in southern Afghanistan on Tuesday, the military alliance said, taking the overall death to

Two NATO troops, an Afghan soldier and a US civilian, were also injured, NATO's International Security Assistance Force (ISAF) said in a statement.

"The cause of the crash is under investigation," it said. "There are no reports of enemy fire in the area."

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It gave no details on the nationalities of those killed or the exact location of the crash.

The four injured people had been taken to an ISAF medical facility, the statement said.

The crash has raised the death toll among foreign troops in Afghanistan this year to 529, according to an AFP toll based on the icasualties.org website, surpassing the figure of 521 for 2009.

A total of 2,097 coalition troops have now died since the US-led invasion of 2001 which ousted the Taliban regime.