A helicopter carrying civilian contractors working for foreign troops in Afghanistan crashed on Tuesday in the south of the country killing at least six people on board, NATO said. It was not immediately clear why the helicopter came down in the Sangin district of Helmand province, where US Marines, British and Afghan forces are pressing assaults against Taliban strongholds in the run-up to key elections. 'At around 9:00 am (0430 GMT) this morning, a private helicopter has crashed outside Sangin military base... we have at least six people killed,' said a spokesman for the NATO-led International Security Assistance Force (ISAF). 'The passengers were all civilians,' he added, speaking from the biggest NATO base in southern Afghanistan at Kandahar. Another ISAF spokesman said the aircraft was carrying civilian contractors, but their nationalities were not immediately known and it was not clear how many people were on board. Fazul Haq, head of the local administration in Sangin, told selfnews that he saw the chopper catch fire and crash. 'I was on the rooftop of a building when I saw a helicopter... Suddenly, about two kilometres from a foreign forces' base in Sangin I saw the chopper catch fire, then crash down,' he said. 'Even now I can see flames and smoke,' he added, speaking to selfnews by telephone from the remote southern district. A spokesman for the Taliban, which is waging an increasingly deadly insurgency against Western troops and the Afghan government, claimed that its fighters shot down the helicopter. The Taliban frequently make claims and exaggerate their prowess in statements that are impossible to confirm. 'It was a Chinook helicopter and we brought it down,' the spokesman, Yousuf Ahmadi, told selfnews by telephone. The incident comes one week after a British and two Canadian soldiers were killed when their Ch-146 Griffon helicopter crashed on take-off in Afghanistan. Canadian military officials said the crash on July 6, also in the south of the country, was likely caused by mechanical failure, not enemy fire. Afghanistan has seen a surge of violence in recent weeks, as the country prepares to go to the polls for landmark presidential and provincial council elections on August 20. With the nearly eight-year insurgency at its deadliest, the United States has dispatched up to an extra 21,000 soldiers in a bid to stabilise the country ahead of the vote. Military casualties have surged in recent weeks as about 4,000 US Marines and thousands of British and Afghan forces battle their way into Taliban strongholds in the south in separate assaults launched about three weeks ago. There are about 90,000 international troops, mainly US, British and Canadian, deployed in Afghanistan to help Kabul defeat the Taliban insurgency which followed the 2001 US-led invasion to oust their Kabul regime. 