The medical charity MSF says at least three of its staff were killed in the Afghan city of Kunduz after a clinic was hit by an air strike on Saturday.
US forces were carrying out air strikes at the time. The Nato alliance has admitted the clinic may have been hit.
MSF says more than 30 staff are unaccounted for. The hospital had 105 patients at the time.
There has been intense fighting in Kunduz since Taliban fighters swept into the northern city on Monday.
It was the first major urban centre to fall to the Taliban in 14 years. Medecins Sans Frontieres (MSF) said its clinic was hit several times during “sustained bombing and was very badly damaged” at 02:10 local time (22:40 GMT) on Saturday.
A spokesman for US forces in Afghanistan, Col Brian Tribus, said: “US forces conducted an air strike in Kunduz city at 02:15 (local time)… against individuals threatening the force.
“The strike may have resulted in collateral damage to a nearby medical facility.”
The incident is being investigated, he added.
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.”
The charity says it does not have final figures for the dead and injured. However, it says when the bombardment took place, 105 patients and their caretakers were in the hospital, along with more than 80 MSF staff.
It is the only medical facility of its kind in the north-east of Afghanistan. Afghan officials said the government had regained control of the city on Friday, but the Taliban denied the city had been retaken.
Eyewitnesses said they saw Taliban fighters on the streets or hiding in civilian houses.
Taliban leader Mullah Akhtar Mansour had described the seizure of Kunduz as a “symbolic victory”.
Kunduz, with a population of around 300,000, is one of Afghanistan’s largest cities and strategically important both as a transport hub and a bread-basket for the region. (BBC)