A separate bomb attack killed three young nephews of a high-profile ally of President Hamid Karzai in the southern city of Kandahar, where violence has surged ahead of a planned major offensive by US-led forces.

Afghan and NATO officials declined to give details of the cause of the blast at the base near Kabul airport, saying it was still under investigation.

Taleban spokesman Zabihullah Mujahid said the attack was carried out by a Taleban infiltrator, who struck while foreign advisers were training Afghan troops.

The airport is a combined civil-military one which is also used by foreign troops and in the past has come under rocket attacks as well as suicide strikes at its gates.

A Defense Ministry spokesman Zaher Murad said one Afghan soldier was killed and three wounded. The NATO-led International Security Assistance Force said in a statement that one of its members was also killed and several wounded.

NATO efforts to train Afghan forces have been plagued in recent months by "rogue" Afghan soldiers and police turning weapons on their trainers, and infiltrators gaining access to compounds meant to be secure.

In November, an Afghan police trainee killed five British soldiers at a checkpoint. In December, a Jordanian infiltrator at a US base killed seven employees or contractors working for the CIA and a Jordanian intelligence officer.

In the other big attack on Monday, a bomb strapped to the back of a donkey killed three young nephews of a tribal chief allied to Karzai in Kandahar, the president's home town.

Three pedestrians and two police were also wounded in the blast, which struck a police post guarding the home of the tribal chief, Fazluddin, said Zalmai Ayoubi, spokesman for the regional governor.

The city, birthplace of the resurgent Taleban, has witnessed a dramatic upsurge in violence in recent weeks, ahead of a planned major offensive by US-led NATO troops.

US officials say their operation in Kandahar in coming months will be the centerpiece of a military campaign this year to turn the tide in the eight-year-old war with an extra 30,000 troops sent by President Barack Obama.