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 (1030 PST) 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.