Aid organization insists it gave U.S. and Afghan authorities exact location of the hospital three days before the strike

'Patients burned to death in their beds,' MSF chiefs on the ground say

MSF says bombing continued for 30 minutes after U.S. military was informed the hospital had been hit

Three children are among the dead, with 37 people seriously injured

Médecins Sans Frontières said 12 medics and seven patients were killed

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

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

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

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A U.S. airstrike in Afghanistan has killed at least 19 people, including 12 medics with the aid organization Doctors Without Borders. Above, a photo of the trauma center

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

Investigation: President Obama has pledged to discover the exact circumstance behind the bombing on the Afghan hospital on Saturday that left 19 dead including 12 medics

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 present in the hospital.

The charity said despite frantic calls to American and Afghan military officials in Kabul and Washington, the attack continued for another 30 minutes, with the main hospital building housing the intensive care unit and emergency rooms being targeted.

'The bombs hit and then we heard the plane circle round,' said Heman Nagarathnam, MSF's head of programmes in northern Afghanistan.

'There was a pause, and then more bombs hit. This happened again and again. When I made it out from the office, the main hospital building was engulfed in flames.

'Those people that could had moved quickly to the building's two bunkers to seek safety. But patients who were unable to escape burned to death as they lay in their beds.'

In a statement released by the White House, US President Barack Obama offered his 'deepest condolences' for what he called a 'tragic incident'.

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

The medical aid organization said that there were still 30 people unaccounted for as of early Saturday morning local time

The US airstrike on a Doctors Without Borders hospital in Afghanistan may have been a war crime, according to UN human rights chief Zeid Ra'ad Al Hussein (pictured)

The Afghanistan insurgency has often been in southern provinces close to Kabul and the Pakistan border, though the Taliban's latest move occurred in the north

NATO earlier conceded US forces may have been behind the bombing, after forces launched a strike 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 NATO 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 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'.

'This event is utterly tragic, inexcusable and possibly even criminal,' he said.

Doctors Without Borders staff in shock today following the bombings, in one of the remaining parts of the hospital in Kunduz, Afghanistan

MSF surgeons work in an undamaged part of the hospital. Medical staff are said to be 'working round the clock' to cope with the injuries and deaths caused by the bombing, which damaged the emergency room and operating theater

MSF said some 105 patients and their caregivers, as well as more than 80 international and local MSF staff, were in the hospital, the only medical facility in the area that can deal with major injuries, at the time of the bombing.

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

'This attack is abhorrent and a grave violation of International Humanitarian Law,' said MSF President Meinie Nicolai.

'We demand total transparency from Coalition forces. We cannot accept that this horrific loss of life will simply be dismissed as 'collateral damage'.'

Doctors Without Borders said that it had been flooded with patients after fighting broke out in Kunduz. Above, a picture of the hospital on Wednesday

Fires burn in part of the MSF hospital after it was hit by an airstrike, in a photo released by MSF. The charity previously said that it had treated 394 people in the city since Monday, adding that 105 patients and 80 staff were present at the time of the attack - including 59 children

A member of the Afghan security forces stands near an armoured military vehicle today in Kunduz. The hospital has been running 'beyond capacity' amid heavy fighting in Kunduz, which saw the nothern city fall into Taliban hands for several days

An injured civilian is taken to safety after a battle with the Taliban today in the city of Kunduz. Government ministers said that hundreds of Taliban fighters had come into the city during the Eid holiday weekend before launching their attack Monday

Kunduz is facing a humanitarian crisis, with thousands of civilians caught in the crossfire between government forces and insurgents. At least 60 people are known to have died and 400 wounded in recent fighting.

The Taliban seized on the incident, saying 'barbaric American forces... carried out deliberate air strikes on a civilian hospital'.

In a statement, it denied any of its fighters were being treated at the MSF clinic 'because the prevailing military situation of Kunduz would not allow us to admit our patients to the said hospital'.

The Islamists' offensive in Kunduz marks a major blow for Afghanistan's Western-trained forces.

US-led NATO forces ended their combat mission in Afghanistan last December, though a 13,000-strong force remains for training and counter-terrorism operations.