Two US service members were shot dead today by another Westerner inside the Afghan Interior Ministry, officials said.

The attack came as tensions between the Afghans and the Americans are high following the burning of copies of the Muslim holy book at a US base that sparked days of deadly protests. But Afghan officials said Saturday's shooting did not involve any Afghans.

Instead, they said, another international adviser turned his gun on two Americans, two Afghan officials said, speaking anonymously to discuss a NATO incident. One of the officials noted that the shooting occurred inside a secure room at the ministry that Afghan staff do not have access to.

A US official in Washington confirmed that the two killed were American and that they were killed by "an assailant," without giving further information about the attacker. The official spoke anonymously to discuss information that had not been publicly released.

NATO forces have advisers embedded in many Afghan ministries, both as trainers and to help manage the transition to Afghan control and foreign forces prepare to withdraw by the end of 2014. The Afghan Interior Ministry oversees all of the country's police, so has numerous NATO advisers.

The shooting comes on the fifth day of protests across the nation sparked by the burning of Qurans at a US base.

At least 25 people have been killed and hundreds wounded since Tuesday, when it first emerged that Qurans and other religious materials had been thrown into a fire pit used to burn garbage at Bagram Air Field, a large US base north of Kabul.

President Barack Obama and other US officials apologised and said it was a terrible mistake, but the incident has sent thousands to the streets in this deeply religious country.