A suicide car bomber has struck a Nato convoy in Kabul, killing at least 17 people, Afghan officials said.
Nato said 13 service members, all of them American, were killed.
Afghan officials had said earlier that four Afghans, including a policeman, were killed.
The Taliban claimed responsibility for the attack, which took place near Darulaman Palace, the bombed-out seat of former Afghan kings on the south-west outskirts of the capital.
The attack was the deadliest of three separate incidents which targeted either the US-led coalition or Afghan government offices in the country.
In the volatile south, an area traditionally viewed as the Taliban's stronghold, Nato said a man in an Afghan military uniform turned his weapon on coalition and Afghan forces, killing two.
The gunman was also killed, the coalition said in a statement but provided no other details.
The bombing on the outskirts of Kabul appeared to target a Nato convoy which included a military bus.
Nato and Afghan forces sealed off the area as rescuers rushed around the attack site, according to a reporter at the scene.
Two Nato helicopters landed to airlift casualties while coalition troops using loudspeakers ordered bystanders to evacuate the area. Heavy black smoke poured from the bus, which was engulfed in a fireball.