Image caption So-called "insider attacks" accounted for a growing proportion of Isaf casualties last year

Two US soldiers in Afghanistan have been shot dead in a so-called insider attack, US and Afghan sources say.

A number of Afghan troops also died in the shooting at a remote military base in Wardak province, not far from Kabul.

The US military called the attack a "betrayal". The killer - a member of the Afghan security forces - was shot dead at the scene.

Last year more than 60 Nato troops were killed by Afghan security personnel or insurgents posing as them.

The gunman - a member of the Afghan army or police - opened fire as US special forces and Afghan commandos held an early morning meeting.

Another 10 US troops were wounded, the US military said. The number of Afghan casualties has not been made public.

The attacker was also killed and the US-led special operations task force said the area had been secured.

Wardak province is the scene of particular tension between the Afghan authorities and US troops fighting Taliban militants.

Afghan President Hamid Karzai had ordered US special forces to leave the province by the middle of this month because of allegations of torture and disappearances carried out by Afghan troops working with them.

Tension

Earlier on Monday, police in Kabul said two civilian lorry drivers were killed and one wounded when they were fired on by an Isaf convoy.

The international security force Isaf said soldiers had opened fire to protect themselves when the two drivers failed to comply with a warning.

The latest deaths come amid continuing tension between the Afghan government and the US, as Nato troops prepare to end combat operations next year.

President Karzai has been strongly critical of US and allied forces for causing civilian casualties.

So-called "insider attacks" by members of the Afghan security forces, or Taliban infiltrators posing as them, have accounted for a growing proportion of Isaf casualties.