KABUL (AFP) - Afghan forces called in a US air strike on a Kunduz hospital that killed 22 people, the top American commander in Afghanistan said Monday, after medical charity MSF branded the incident a war crime.

General John Campbell's statement marks the first US military acknowledgement it was behind Saturday's devastating raid in the northern Afghan city which triggered international outrage.

But his remarks prompted Doctors Without Borders (MSF) to blast the "discrepancies" in US accounts of the strike, which caused patients to burn to death in their beds and reduced the hospital to smouldering rubble.

Campbell's statement was at odds with previous US military claims that the strike was carried out to protect American special forces on the ground from enemy fire.

"We have now learned that on October 3, Afghan forces advised that they were taking fire from enemy positions and asked for air support from US air forces," Campbell told reporters.

"An air strike was then called to eliminate the Taleban threat and several civilians were accidentally struck." There was no immediate reaction from Afghan officials but they have previously claimed that insurgents were using the hospital as a position to target soldiers and civilians.

Campbell's remarks come as pressure mounts on Washington to come clean over the strike, which occurred days after the Taleban overran Kunduz in their most spectacular victory in 14 years.

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

MSF, which has closed its trauma centre seen as a lifeline in the war-battered region, accused Campbell of passing the buck.

"Their description of the attack keeps changing - from collateral damage, to a tragic incident, to now attempting to pass responsibility to the Afghanistan government," MSF General Director Christopher Stokes said.

"The reality is the US dropped those bombs," he said. "The US hit a huge hospital full of wounded patients and MSF staff.

"With such constant discrepancies in the US and Afghan accounts of what happened, the need for a full transparent independent investigation is ever more critical."

The group said Afghan and coalition troops were fully aware of the exact location of the hospital, having been given GPS coordinates of the facility which had been providing care for four years.

It added that 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.

Mr Stokes earlier also hit out at claims by Afghan officials that militants were inside the hospital.

"These statements imply that Afghan and US forces working together decided to raze to the ground a fully functioning hospital with more than 180 staff and patients inside because they claim that members of the Taleban were present," he said. "This amounts to an admission of a war crime."

US Defence Secretary Ashton Carter earlier expressed sadness over the "tragic loss of life" and repeated President Barack Obama's promise of a full and transparent investigation.

Mr Stokes, however, stressed the need for an independent probe, saying "an internal investigation by a party to the conflict would be wholly insufficient".

UN rights chief Zeid Ra'ad Al Hussein has also called for the incident to be "promptly, thoroughly and independently investigated", noting that "an air strike on a hospital may amount to a war crime". But a spokesman for UN Secretary General Ban Ki-moon said Monday it was "still early days".

"We are waiting to see what comes out of the official US, NATO and probably Afghan investigations if we move a little further," said Stephane Dujarric.

"We are looking for a credible and transparent investigation," he said. "That's all we can say today."

The air raid came five days after Taleban fighters seized control of the strategic northern city of Kunduz, in their biggest success since being toppled by a US-led coalition in 2001.

Residents said Monday that clashes in the city appeared to have subsided.

Afghan forces, backed up by their NATO allies, claim to have wrestled back control of Kunduz, where decomposing bodies still littered the streets.

At least 60 people have died and 400 have been wounded in the past week's fighting.

Saturday's raid left the hospital's main building completely gutted. Some of the bodies of those trapped inside were charred beyond recognition.

The dead included 12 MSF staff and 10 patients, among them three children.

MSF's withdrawal from Kunduz comes as the region grapples with a humanitarian crisis, with food and medicine shortages affecting thousands of civilians caught in the crossfire between government forces and insurgents.

The hospital was the only medical facility in the whole northeastern region of Afghanistan that can deal with major war injuries.