Kabul (AFP) - Doctors Without Borders upped the death toll from a US strike on its Kunduz hospital to 42 Saturday, as a new UN report laid bare grim human rights violations during the Taliban's stunning capture of the Afghan city.

Pressure is growing for an international inquiry into the catastrophic October 3 bombing of the hospital which came as NATO-backed Afghan forces clashed with insurgents for control of the northern provincial capital.

MSF released the new toll as a UN report on Saturday more than trebled the number of civilians killed after the Taliban overran the city, causing "extreme suffering" for residents trapped by two weeks of fighting.

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

"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 included 14 MSF staff, 24 patients and four caretakers, it said.

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".

The charity has called for an independent investigation, delivering a petition signed by 547,000 people to the White House Wednesday, but so far neither the US nor Afghanistan has agreed.

Guilhem Molinie, the charity's Afghanistan director, said those killed had given their lives treating people trapped in a war zone.

"We ask President Obama to honour their work and their memory by consenting to a truly independent, international investigation," he said in a statement released by MSF Saturday.

- Chaos, criminality, killings -

The special report from the United Nations Assistance Mission in Afghanistan (UNAMA) detailed a breakdown of rule of law during the Taliban's occupation of Kunduz that resulted in "a loss of protection of the most basic human rights".

The catalogue of violence and destruction cast a light on the "extreme suffering" of the residents of Kunduz and offered an ominous blueprint of what could happen should the insurgents ever return to power.

Chaos in the city "enabled an environment in which civilians were subjected to arbitrary killings, assault, other forms of violence, including gender-based violence, threats and widespread criminality", the report stated.

It provided a "preliminary" figure of 848 civilian casualties -- 289 deaths and 559 injuries -- that occurred in Kunduz province between September 28, when the Taliban launched the assault on the city, and 13 October, when they retreated.

"The vast majority of these casualties resulted from ground fighting that could not be attributed solely to one party," it stated, warning that the figures could yet rise.

The new figures did not include the additional toll from MSF.

The document confirmed earlier reports that the Taliban had targeted women's rights activists, government workers and journalists in the city, and cited claims they executed civilians and used child soldiers during the offensive.

It also described thousands of civilians trapped in a "dire" humanitarian situation, hiding in their own houses for days amid food shortages and without electricity or clean drinking water, unable to flee for fear they would be shot by snipers.

The anarchy allowed "criminal elements not directly associated with the Taliban" to engage in revenge killings, looting and other forms of violence, it said.

UNAMA also cited reports of the desecration of bodies by both the Taliban and Afghan forces during the offensive, saying that mutilation of the dead "may amount to a war crime".

The report was published online at: http://unama.unmissions.org/Portals/UNAMA/civcas/Special%20Report%20on%20Kunduz%20province_12%20December%202015.pdf