A total of eight NATO soldiers were killed in Afghanistan on Saturday -- the bloodiest day this year for international forces -- the coalition said, as a further death was announced.

In a statement issued on Sunday, the German army confirmed that one of its special forces soldiers was killed in an insurgent attack during a joint operation near the northern city of Baghlan.

Also on Saturday, five US troops died in a Taliban roadside bomb blast and two other NATO troops were shot dead by an Afghan soldier in an "insider attack".

The nationality of the two was not released, in line with coalition policy.

The five US troops were killed when their vehicle hit a powerful improvised explosive device in the southern province of Kandahar, local police said.

Akram Khpalwak, governor of the western province of Farah, said the victims of the insider attack died when an Afghan soldier opened fire in Bala Buluk district.

The Taliban vowed a spate of insider attacks when they launched their spring offensive a week ago, but NATO says that most such shootings stem from personal grudges and cultural misunderstandings rather than militant plots.

The militants' annual offensive opened a crucial period for Afghanistan as its security forces take the lead in offensives against insurgents who are fighting to topple the US-backed government.

All NATO combat missions will finish by the end of next year, and the 100,000 foreign troops deployed across Afghanistan have already begun to withdraw from the battlefield.