A suicide bomber has attacked a bus carrying foreign troops in the Afghan capital Kabul killing 13 US soldiers.
A Nato spokesman said the attacker detonated a car filled with explosives.
The attack, in the Darulaman area, west of Kabul, also killed three Afghan civilians and a police officer.
The Taliban has admitted carrying out the attack - one of the worst ground attacks against foreign troops since the beginning of the war. Such attacks are rare in heavily-guarded Kabul.
Separately, three Australian soldiers were killed by a man in Afghan army uniform. Nato said the gunman was also killed in that attack in the south of the country.
Fewer attacks have hit Kabul this year, compared to 2010.
But the Taliban and the Pakistan-based Haqqani network, which is linked to it, have still been able to penetrate the city's defences.
In September, the Haqqani network launched an attack on the American embassy and Isaf headquarters which lasted 20 hours.
Vehicles burning
The attack happened near Darulaman Palace, the bombed-out seat of Afghanistan's former kings on the south-west outskirts of the capital.
The suicide attacker driving a red Toyota Corolla detonated the bomb just as the bus was passing at 11:20 (06:50 GMT). The armoured bus was blown over by the force of the explosion.
Nato helicopters were seen taking away casualties.
"It was a very strong bomb," said one eyewitness, Gulam Saki.
Thick black smoke rose from the scene - and some distance away there were shattered windows and scattered pieces of twisted metal, suggesting the blast had been massive.
"A suicide car bomb attack was carried out on a bus of foreign forces in the Darulaman area of Kabul," Taliban spokesman Zabiullah Mujahid said in a text message to AFP news agency.
"Initial reports indicate that 13 International Security Assistance Force (Isaf) service members died following an improvised explosive device attack in Kabul earlier today," the force said in a statement. US officials later confirmed the soldiers were American.
It was the deadliest incident since the Taliban shot down a US helicopter in August, killing 30 US special forces.
There has been an increase in militant attacks across Afghanistan in recent months, despite the presence of more than 130,000 foreign troops.
But this was a very significant attack, says the BBC's Quentin Sommerville in Kabul. It is rare for so many servicemen to die in a single incident, our correspondent says.
On Thursday the Taliban said it was behind an attack on a compound housing Western officials and military personnel in the city of Kandahar. The attack was repelled after a 15-hour battle.
The US is planning to withdraw its troops from Afghanistan and hand over security to local forces by 2014.
