A suicide bomber has struck a Nato convoy near the southern Afghan city of Kandahar, the US military said, adding that there were casualties.
The Taliban took responsibility for the attack and a spokesman for the insurgents said the bombing killed 15 soldiers - a claim that appeared exaggerated, as many similar claims have been in the past.
Military spokesman Lt Damien E Horvath could not say how many casualties there were, or provide their nationalities.
The Nato mission, known as Resolute Support, "can confirm that a Nato convoy was attacked in Kandahar. The attack did cause casualties", he said.
In their claim of responsibility, the Taliban also said the attack destroyed two armoured tanks.
The insurgents' spokesman for southern Afghanistan, Qari Yusuf Ahmadi, said fighter Asadullah Kandahari was the "hero" who carried out the attack with a small pick-up truck, packed with explosives.
Kandahar province was the Taliban spiritual heartland and the headquarters of their leadership during their five-year rule, which ended with the US invasion in 2001.
Ghulam Ali, who runs a mechanics shop near the attack site on the outskirts of the city of Kandahar, said the intensity of the blast knocked him out.
When he came to, he saw a military vehicle on fire on the road.
He stepped out of his shop but a sudden burst of gunfire drove him back inside, he said.
Helicopters arrived and he saw soldiers being taken away from the scene but could not determine the extent of their injuries.
Shah Agha Popal, who runs a vehicle parts shop nearby, said he also saw soldiers being taken away by two helicopters.
"But I couldn't tell if they were wounded or if they were dead," he said.
The combined US and Nato troop contingent currently in Afghanistan is about 13,500.
The Trump administration is deciding whether to send about 4,000 or more US soldiers to Afghanistan in an attempt to stem Taliban gains.
The attack came as Afghan authorities in western Herat province tightened security ahead of a mass funeral for the victims of an attack the previous evening that killed 29.
A suicide attacker opened fire inside a mosque packed with worshippers at evening prayers, before detonating his explosives.
A second explosion came 10 minutes later.
No-one has claimed responsibility for that attack, but it came a day after Islamic State warned it would strike Shiites.
The Sunni militant group considers Shiite Muslims as apostates.
Herat provincial spokesman Jilani Farhad said that to reduce the possibility of more attacks, a planned Shiite protest against the attack was to be held just before the burial, rather than at a separate time and location.
Along with the 29 killed, 64 people were wounded, 10 of them critically.