KABUL (AFP) Nine NATO troops were killed in a helicopter crash in the insurgent heartland of southern Afghanistan on Tuesday, making 2010 the deadliest year for international forces since the war began. The Taliban, who have been waging an increasingly deadly insurgency against the Afghan government and foreign troops since the 2001 US-led invasion ousted them from power, immediately claimed responsibility. The NATO-led International Security Assistance Force (ISAF) said the cause of the crash was under investigation, adding: There are no reports of enemy fire in the area. The helicopter came down in the Daychopan district of Zabul province, said provincial spokesman Mohammad Jan Rasoulyar. We dont know the cause of the crash or the number of casualties, he said. Taliban spokesman Yousuf Ahmadi said the insurgents shot the helicopter down and killed more than 10 foreign soldiers. The militia routinely exaggerate their claims. Another NATO soldier, an Afghan soldier and a US civilian were injured, ISAF said, but it did not identify the nationalities of the dead troops. CNN television said the dead were Americans, quoting on its website an unnamed Western defence source. But ISAF was unable to comment and said the nationalities and other details would be released by the country of origin. We have no more details at the moment because the helicopter recovery operation is underway, an ISAF officer told AFP on condition of anonymity. The incident brings to 529 the number of foreign troops killed this year, according to an AFP tally based on the count kept by icasualties.org, surpassing the previous record of 521 deaths in 2009. A total of 2,097 coalition troops have now died since the US-led invasion of 2001, which ousted the hardline Taliban regime and set off a brutal insurgency which has also killed thousands of Afghans. After only nine months, 2010 has now become the deadliest year of the long war, with the extra deployment of international forces to nearly 150,000 drawing more battlefield engagements and leading to a spike in casualties. June was the deadliest month of the war for coalition troops, with 103 fatalities, the tally shows. Ten foreign troops, mostly American, were killed in a series of attacks across the country on June 22, and another 10 ISAF soldiers died in bombings in the south and east on June 7. Until recently NATO identified American casualties, while leaving coalition partners to identify their own dead separately. That changed earlier this month and US casualties are no longer revealed. The Americans are bearing the heaviest burden of the war, accounting for at least 20 of this months 39 troop deaths, according to icasualties.org. Meanwhile, NATO said an airstrike and ground clashes killed 14 insurgents in an operation in Helmands volatile Nad Ali district on Monday. Taliban influence is strongest in the south and the eastern provinces which border Pakistan.