Obama pledges probe into fatal airstrike on Afghan hospital

President Barack Obama has promised a full investigation into an apparent US air strike on an Afghan hospital that killed 19 people, a bombing which the UN said could amount to a war crime.

Doctors Without Borders (MSF) said patients burned to death in their beds during a raid that continued for more than an hour early Saturday, even after US and Afghan authorities were informed the hospital had been hit.

The air raid came days after Taliban fighters seized control of the strategic northern city of Kunduz, in their most spectacular victory since being toppled from power by a US-led coalition in 2001.

Afghan forces, backed up by their NATO allies, claimed to have wrestled back control of the city.

But the defence ministry in Kabul said "a group of armed terrorists... were using the hospital building as a position to target Afghan forces and civilians".

MSF has denied any combatants were in the hospital.

The charity said that despite frantic calls to military officials in Kabul and Washington, the main building housing the intensive care unit and emergency rooms was "repeatedly, very precisely" hit almost every 15 minutes for more than an hour.

US President Barack Obama offered his "deepest condolences" for what he called a "tragic incident".

NATO earlier conceded US forces may have been behind the bombing, after its forces launched a strike which they said was intended to target militants.

The incident has renewed concerns about the use of US air strikes in Afghanistan, a deeply contentious issue in the 14-year campaign against Taliban insurgents. 

UN rights chief Zeid Ra'ad Al Hussein called for a full and transparent probe, noting: "an air strike on a hospital may amount to a war crime."