Three US contractors were among 12 killed by a Kabul bomb that targeted their Nato convoy, the U.S.-led coalition said, making the attack one of the most lethal for US citizens in Afghanistan in recent years.

The suicide attack outside a hospital on a residential street at rush hour killed mainly Afghan civilians and injured scores more, heightening the anger felt in Kabul after a barrage of deadly blasts this month killed dozens.

The attacker drove his car towards an armoured pick-up truck belonging to contractor DynCorp International which was torn open and left twisted and blackened by the blast. Dozens of vehicles were destroyed, including a school van.

One US contractor died when the bomb exploded and two died from their wounds.

The US embassy in Kabul condemned the bombing.

"The United States remains committed to assisting our Afghan partners in their efforts to ensure a peaceful future," it said.

Contractors such as DynCorp, which has a long involvement in the war and provided bodyguards for the last president, have frequently been targeted. Four US DynCorp employees were killed in a similar Kabul suicide attack in 2013.

Bombings have increased in Kabul since the government and the Taliban in July confirmed Taliban leader Mullah Omar died two years ago, putting paid to hopes that the insurgents would quickly return to the negotiating table.

The UN mission in Afghanistan called for an "immediate halt to all such disproportionate attacks" in civilian-populated areas.

Fighting killed 5,000 civilians in the first half of this year, more than at any point since war started in 2001.

The violence has strained Afghanistan's ties with Pakistan, with Afghan President Ashraf Ghani accusing the neighbouring country of not doing enough to stop militants planning attacks from training camps that he says lie across the border.

Pakistan called the latest attack on civilians "cowardice" and promised to fight against "a common enemy".