A roadside bomb killed five American troops in southern Afghanistan on Thursday, the U.S. military said, less than a week after American forces suffered their worst single loss of the Afghan war when Taliban insurgents shot down a helicopter.

The U.S. military and the NATO-led International Security Assistance Force (ISAF) did not give details regarding Thursday's incident. A Pentagon spokesman in Washington said the victims were Americans.

At least 50 foreign troops have been killed so far in August.

Violence is at its worst in Afghanistan since U.S.-backed Afghan forces toppled the Taliban government in late 2001, with record civilian casualties and high levels of foreign troop deaths during the first half of 2011.

A Chinook troop-carrying helicopter crashed five days ago in central Afghanistan after being hit by what U.S. officials believe was a rocket-propelled grenade, killing 30 U.S. troops, seven Afghan forces and one Afghan civilian interpreter.

ISAF said on Wednesday that it had killed the Taliban militants responsible for shooting down the helicopter.

The Pentagon on Thursday released the names of the U.S. forces killed, confirming that 17 of them were Navy SEALs and an additional five were special operations forces attached to the SEALs. There were also three special operations forces from the Air Force on the helicopter.

Overnight, Taliban insurgents attacked a police checkpoint in southern Helmand province, killing five Afghan policemen, Helmand police chief Abdul Hakim Angar said. No further details of the incident were available.

One foreign soldier was killed by a roadside bomb in southern Afghanistan on Wednesday, ISAF said, giving no further details.

The spike in casualties -- almost 390 foreign troops have been killed so far this year, compared with a record 711 in 2010 -- comes at a time of growing unease about the increasingly unpopular and costly war.

Afghan security forces have been hit even harder than foreign troops. A total of 1,292 Afghan police and 821 Afghan soldiers were killed last year, said the Afghan government.

But it is civilians who have borne the brunt of the war. U.N. figures show a record 1,462 Afghan civilians were killed in conflict-related incidents in the first six months of 2011.

Last month foreign troops began the first phase of a gradual process to hand security control to Afghan soldiers and police. The handover is due to be completed by the end of 2014.

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