Kunduz, Afghanistan (BBN)-Medical charity MSF says three of its staff were killed when its clinic in the Afghan city of Kunduz was hit by sustained bombing.
It said another 30 people were unaccounted for. There is no word on the nationalities of the victims, reports BBC.
A Nato statement says a US air strike “may have resulted in collateral damage to a nearby medical facility”.
The Taliban has clashed with government troops since its militants seized the northern city earlier this week.
Kunduz was the first major urban centre to fall to the Taliban in 14 years.
The medical clinic run by Medecins Sans Frontieres (MSF) was hit several times during “sustained bombing and was very badly damaged” at 02:10 local time (22:40 GMT) on Saturday.
A statement from Nato said: “US forces conducted an air strike in Kunduz city at 02:15 (local time)… against individuals threatening the force”. But, it continues, the strike may have caused damage to a nearby medical facility.
Nato says it is investigating.
MSF director of operations Bart Janssens said: “We are deeply shocked by the attack, the killing of our staff and patients and the heavy toll it has inflicted on healthcare in Kunduz.”
Nato planes have been backing government efforts to retake the city in recent days.
Many Taliban fighters have been pushed back to the outskirts, but fierce battles are continuing near the airport.