Bomb kills seven US soldiers in southern Afghanistan

NATO confirmed the deaths but however declined to disclose the nationalities. Earlier, a Nato helicopter had also crashed in eastern Afghanistan, killing one soldier, officials confirmed.

Almost 200 foreign troops have been killed by militants in Afghanistan so far this year alone.

The BBC reports that, according to Tafsir Khogyani, commander of the border police for southern Afghanistan, “Coalition and Afghan police went to destroy a Taliban container which had some explosives and weapons. As soon as the helicopter landed and other Isaf forces were patrolling, the explosion took place.”

"It took place in Shorabak district 19km (12 miles) from the Pakistani border," he was also reported as saying.

Spin Boldak police chief Gen Abdul Raziq was also reported as saying that insurgents had been cleared from the area only two months ago. Shorabak district lies between the districts of Registan and Spin Boldak.

In the meantime, NATO-led forces are saying that they have pushed back Taliban fighters in a district in the eastern province of Nuristan. However, local officials said that up to 500 insurgents seized the district of Doab on Wednesday.

The international security mission disputes the account given by local leaders. "At no point was the district centre overrun, the Taliban were never in control," said Isaf spokesman Maj Tim James.

Afghan and Nato troops arrived in the district on Wednesday afternoon and carried out a number of air strikes, he said. "The area remains calm with Afghan and Nato troops very much in control," he added.

Nato's claims or how far their control extends in the district is currently hard to dispute or confirm. The provincial governor says nearly 30 insurgents were killed on Wednesday - questions remain over the whereabouts of hundreds of others.