KABUL • US 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 United Nations said could amount to a war crime.

"The Department of Defence has launched a full investigation, and we will await the results of that inquiry before making a definitive judgment as to the circumstances of this tragedy," Mr Obama said.

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

"Twelve staff members and at least seven patients, including three children, were killed; 37 people were injured," the medical charity said. "This attack is abhorrent and a grave violation of international law."

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.

"The bombs hit, and then we heard the plane circle round," said Mr Heman Nagarathnam, MSF's head of programmes in northern Afghanistan. "There was a pause, and then more bombs hit. This happened again and again."

The charity said yesterday that it had withdrawn its staff from Kunduz. The hospital there was the only medical facility in the whole north-eastern region of Afghanistan that could deal with major war injuries and its closure, even temporarily, could have a devastating impact on local civilians.

"The MSF hospital is not functional any more. All critical patients have been referred to other health facilities and no MSF staff are working in our hospital," said spokesman Kate Stegeman. "I can't confirm... whether our Kunduz trauma centre will reopen or not."

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

Afghan forces, backed by their Nato allies, claim to have wrestled back control of the city.

But the Defence Ministry in Kabul said "a group of armed terrorists... was using the hospital building as a position to target Afghan forces and civilians".

The medical aid group denied this. "The gates of the hospital compound were closed all night, so no one that is not staff, a patient or a caretaker was inside the hospital when the bombing happened," MSF said in a statement yesterday. "In any case, bombing a fully functioning hospital can never be justified."

MSF said Afghan and coalition forces were fully aware of the exact location of the hospital, having been given GPS coordinates of a facility it said had been providing care for four years.

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

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

The incident has renewed concerns about the use of US air strikes in Afghanistan, a deeply contentious issue in the 14-year campaign against Taleban 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." He added that the incident "is utterly tragic, inexcusable and possibly even criminal".

AGENCE FRANCE-PRESSE, REUTERS