A military vehicle transports an injured civilian after a battle with the Taliban in the city of Kunduz, Afghanistan October 3, 2015.

An air strike Saturday left three Doctors Without Borders staff dead and dozens more unaccounted for in the embattled Afghan city of Kunduz, the medical charity said, with NATO saying U.S. forces may be behind the attack.

The MSF hospital is seen as a key medical lifeline in the region, which has been running “beyond capacity” in recent days of fighting which saw the Taliban seize control of the provincial capital for several days.

“At 2:10 am (2040 GMT) local time… the MSF trauma center in Kunduz was hit several times during sustained bombing and was very badly damaged,” it said in a statement.

“Three MSF staff are confirmed dead and more than 30 are unaccounted for. The medical team is working around the clock to do everything possible for the safety of patients and hospital staff.”

At the time of the bombing, 105 patients and their caretakers and more than 80 MSF international and national staff were present in the hospital, the charity said.

NATO said U.S. forces may be behind the strike.

“U.S. 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.”

The MSF trauma centre in Kunduz is the only medical facility in the region that can deal with major injuries.

Kunduz has seen heavy fighting since Taliban insurgents stormed the provincial capital on Monday, the first major city to be captured by insurgents since 2001.

Afghan forces, backed by NATO special forces and U.S. air strikes, have been going from house to house in a bid to flush out insurgents in the city.

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