The death toll from a US air strike on an Afghan hospital has jumped to 30, Doctors Without Borders said Sunday, as an initial NATO probe conceded that the catastrophic raid caused civilian casualties.

Three separate investigations are ongoing into the October 3 strike in northern Kunduz city, which sparked a torrent of global condemnation and forced the French-founded charity to close the hospital's trauma center.

"MSF announces with sadness that the death toll is still rising," the charity known by its French abbreviation said in a statement.

"The total number of dead is known to be at least 30, including 10 known patients, 13 known staff, and seven unrecognisable bodies that were in the wreck of the hospital."

It warned that the toll from the strike, which caused patients to burn to death in their beds, could rise still further.

Three investigations -- led by the US, NATO and Afghan officials -- are currently underway as pressure mounts on Washington to come clean over the raid.

NATO on Saturday said its initial probe "determined that the reports of civilian casualties were credible, and we continue to work with the government of Afghanistan to fully identify the victims".

But MSF has repeatedly demanded an independent probe by an international fact-finding commission.

"We need to know the degree to which the lines between military and civilian targets have been blurred," Jason Cone, MSF executive director in the US, wrote in the New York Times on Saturday.

"(Only then) can we better understand the risks our teams in Afghanistan and elsewhere will face."

The attack caused MSF to shut down the trauma center, seen as a lifeline in a war-battered region with scant medical care.

"The destruction of MSF’s 94-bed trauma centre will have a huge impact on access to surgical care for hundreds of thousands of people," the MSF statement said.

"This hospital was the only facility of its kind in northeastern Afghanistan."