The death toll from a devastating US air strike on an Afghan hospital in October has risen to 42, medical charity MSF said Saturday, citing an internal probe as pressure grows for an international inquiry.

The October 3 bombing of the hospital in the northern city of Kunduz during a Taliban offensive forced the facility to close and sparked an avalanche of global condemnation.

The charity has said the raid by a AC-130 gunship lasted nearly an hour and left patients burning in their beds with some victims decapitated and suffering traumatic amputations.

"Previously MSF had reported a death toll of at least 30 people, but the organisation confirms the toll has risen to 42, after methodical review of MSF records and family claims, as well as patient, staff and family testimonies," MSF said in a statement.

"The revised figures include 14 MSF staff members confirmed to have been killed, as well as 24 patients and four caretakers."

The strike was "caused primarily by human error", General John Campbell, the US commander in Afghanistan, said last month, prompting a strong rebuke from the charity who slammed American forces for "gross negligence".

In the aftermath of the strike MSF branded the incident a war crime.

The charity has called for the International Humanitarian Fact-Finding Commission (IHFFC) -- an independent body created under international law but which has never been used -- to investigate the attack.

However, it would need permission from the US and Afghanistan to proceed, and neither country has so far agreed.

MSF on Wednesday delivered a petition signed by 547,000 people to the White House demanding an independent investigation.

"Only a full accounting by an independent, international body can restore our confidence in the commitments of the United States to uphold the laws of war, which prohibit such attacks on hospitals in the strongest terms," Jason Cone, executive director of MSF-USA, said in a statement.

"It is not sufficient for the perpetrators of attacks on medical facilities to be the only investigators."

The strike came after a resurgent Taliban briefly captured the northern provincial capital in their biggest military victory since they were toppled in 2001.

A new UN report on Saturday said the fatalities from the two-week occupation was more than three times higher than previously thought.

"The report provides a preliminary figure of 848 civilian casualties (289 deaths and 559 injured) that occurred in Kunduz province between 28 September and 13 October," said the report.

"The vast majority of these casualties resulted from ground fighting that could not be attributed solely to one party."

The figures did not include the additional toll from MSF.