A roadside bomb killed five US troops Saturday in Afghanistan and two other NATO soldiers were shot dead in an "insider attack", a week after the Taliban launched their spring offensive.

The bomb killed five soldiers in a vehicle in the southern province of Kandahar, local police said, while the two other NATO troops were killed by an Afghan army soldier in the far west of the country.

The nationalities of the two who died in the insider attack was not disclosed by NATO's International Security Assistance Force, in line with coalition policy.

Akram Khpalwak, governor of Farah province, told AFP that an Afghan soldier had opened fire in the Bala Buluk district of Farah, but he was unable to give further details.

General Abdul Razeq, police chief in Kandahar, said that "five American soldiers were killed at about noon when their armoured vehicle hit a powerful roadside mine in Maiwand district."

NATO-led soldiers are fighting alongside Afghan colleagues to thwart Taliban militants, but more than 60 foreign soldiers were killed in 2012 in insider attacks that have bred fierce mistrust between allies.

The last suspected incident was on March 11 when two US soldiers were killed and 10 wounded in Wardak province by a man in Afghan army uniform who also killed several Afghan soldiers.

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 threat has become so serious that foreign soldiers working with Afghan forces are regularly watched over by so-called "guardian angel" troops to provide protection.

The militants' annual spring 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.

More than 11 years after the Taliban regime was ousted in 2001, efforts to seek a political settlement ending the violence have so far made little progress, but pressure is growing ahead of the NATO withdrawal.

US Secretary of State John Kerry on Friday named James Dobbins as the new US special envoy to Afghanistan and Pakistan, saying Dobbins would "continue building on diplomatic efforts to bring the conflict to a peaceful conclusion".

Including Pakistan in any peace negotiations is seen as essential as militants use the border region between the two countries as a safe haven to launch attacks in Afghanistan.

Pakistan, which backed the 1996-2001 Taliban regime in Kabul, is also widely accused of providing covert support for the militants.

The Taliban have rejected holding any peace talks with the Afghan government, saying that President Hamid Karzai is a puppet of the US.

Karzai has clashed repeatedly with the US this year over Afghan sovereignty and the security transition, but he has also been caught up in a scandal alleging that CIA cash delivered to his office was used to buy off warlords.

The president said on Saturday the money -- reportedly packed in suitcases, backpacks and plastic shopping bags -- was used for health care and scholarships, and that full receipts are issued to the Americans.

Anger in Washington over the CIA payments has focused on the cash fuelling endemic corruption that the US and other donor nations say is a prime threat to Afghanistan developing a functioning state system.