The attacker detonated his Toyota Sedan car at 11:20 am (0650 GMT) in the southwest of the city, and the area was now blocked by Afghan and international forces, said police spokesman Hashmat Stanikzai.
At least ten foreign forces, three civilians and a policeman were among the casualties. "The attack targeted an American NATO bus," said a Western military official on condition of anonymity. "There are 10 or 11 people, mostly Americans," he said, giving the death toll.
Spokesman for the interior ministry Siddiq Siddiqui said that three civilians and a policeman had also been killed but he had no information on foreign forces. "It was a huge explosion, I saw at least ten bodies of foreign forces taken out of their capsized bus and evacuated by two helicopters," one witness told AFP at the scene.
Thick black smoke could be seen rising from an inferno still burning at the scene, while fire hoses were putting out another nearby, footage on private television channel Tolo showed. NATO's coalition forces were seen tending to at least once casualty on the ground, while fire trucks and ambulances, sirens blaring, were on their way to the scene where one charred vehicle could be seen lying on its side.
A spokesman for NATO's International Security Assistance Force (ISAF) said the force could not give further details until further information had been gathered.