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.
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The operating theatre and emergency department were among the parts of the hospital complex hit, according to Adil Akbar, a doctor who spoke to the Associated Press news agency.
“I managed to escape after [the] attack but I know that most of the staff and even some of the patients are missing,” he said.
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.
Most of MSF’s staff in Kunduz are Afghan, the charity says.
The BBC’s Emal Pasarly, in Kabul, says fighting has been going on in Kunduz every night, and the area where the hospital is was the centre of the fighting on Friday night into Saturday.
Local people said that helicopters were firing at the hospital, he reports.
A Taliban spokesman said none of their fighters were at the hospital at the time of the bombing.
Afghan officials said the government had regained control of Kunduz 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.
The US-led Nato combat mission in Afghanistan ended in December 2014, but Nato forces remain for training purposes.
Nato’s Resolute Support Mission, which was launched in January 2015, consists of more than 13,000 troops from 42 countries. The US contributes around half of these.