A cloud of dust obscured the northern side of the base immediately after the blast, an AFP reporter on the spot said.

Several ambulances were seen rushing towards the source of the explosion.

The air base, located some 50km north of Kabul, serves as the headquarters for the 12,500-strong US-led coalition.

The base has been attacked several times by rockets and small arms fire.
  
Early last month gunmen fired small arms fire on one of the guard towers of the  base but caused no casualties.