Seven US soldiers have been killed by a roadside bomb in southern Afghanistan, US officials say.
The incident is the deadliest single attack on foreign troops in a month. Nato initially confirmed the deaths but declined to disclose the nationalities.
Earlier, a Nato helicopter crashed in eastern Afghanistan, killing one soldier, officials said.
Almost 200 foreign troops have been killed by militants in Afghanistan so far this year.
Tafsir Khogyani, commander of the border police for southern Afghanistan, told the BBC's Bilal Sarwary in Kabul: "According to our initial information, 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."
Shorabak district lies between the districts of Registan and Spin Boldak.
Spin Boldak police chief Gen Abdul Raziq told the BBC that insurgents had been cleared from the area only two months ago.
Meanwhile, Nato-led forces say they have pushed back Taliban fighters in a district in the eastern province of Nuristan.
Local officials said 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," added Maj James.
It is very hard to confirm Nato's claims or how far their control extends in the district. The provincial governor says nearly 30 insurgents were killed on Wednesday - but questions remain over the whereabouts of hundreds of others.
Our correspondent says the mountainous area has many hiding places.
Officials in Nuristan have been warning for more than a year that the government and Nato should do more to counter the militant threat in the area, he adds.