A suspected US air strike on a hospital in Kunduz Saturday left three Doctors Without Borders staff dead and dozens more unaccounted for, the charity said, days after Taliban insurgents overran the northern Afghan city.

A suspected US air strike on a hospital in Kunduz Saturday left three Doctors Without Borders staff dead and dozens more unaccounted for, the charity said, days after Taliban insurgents overran the northern Afghan city.

The MSF facility is seen as a key medical lifeline in the region and has been running "beyond capacity" during recent fighting that saw the Taliban seize control of the northern provincial capital for several days.

The strike early Saturday left the building engulfed in flames, with photos posted by MSF on Twitter showing shocked and dazed staff in the aftermath of the bombing.

Kunduz has seen heavy fighting since Taliban insurgents overran it on Monday. It is the first major city to be captured by militants since 2001.

MSF said some 105 patients and their caregivers, as well as more than 80 international and local MSF staff were in the hospital at the time of the bombing.

NATO conceded that US forces may have been behind the strike.

"US forces conducted an air strike in Kunduz city at 2:15 am (local time)... against individuals threatening the force," a NATO statement said.

"The strike may have resulted in collateral damage to a nearby medical facility. This incident is under investigation."