It did not give further details of their nationality or what happened in line with policy.

Officials also said that two pilots were injured when a French military helicopter made an emergency landing east of Kabul late on Friday.

The incident happened as the combat helicopter was escorting transport helicopters from a French base in the Surobi district of Kabul province back to the capital itself.

French military spokesman Lieutenant-Colonel Philippe Bou told AFP enemy fire had been ruled out as a cause of the incident, which he said happened during bad weather conditions. An investigation has been launched.

ISAF spokesman Major Tim James added: "I can confirm that an ISAF helicopter made an emergency landing in the Surobi district of Kabul (province) last night. Two pilots were slightly injured."

Independent website iCasualties.org puts the total number of foreign troops killed in Afghanistan this year at 34.

A record 711 foreign troops died in the country in 2010, making it the deadliest year for them since a US-led invasion toppled the hardline Taliban regime in late 2001.

There are around 140,000 international troops in Afghanistan, two-thirds of them from the United States.