A deadly air strike on a Doctors Without Borders (MSF) hospital was "caused primarily by human error", the US commander in Afghanistan said Wednesday, prompting a strong rebuke from the charity who slammed American forces for "gross negligence".

The October 3 air raid on the French charity's hospital during a Taliban offensive in the northern city of Kunduz killed at least 30 people, forcing the facility to close and sparking an avalanche of global condemnation.

The "tragic but avoidable accident (was) caused primarily by human error," General John Campbell told reporters at NATO headquarters in Kabul, adding that those most closely associated with the incident had been suspended from their duties.

He blamed in part fatigue of US troops who had been battling a Taliban offensive in Kunduz for five days, adding that the mistake was "compounded by process and equipment failures".

Responding to the release of the investigation, MSF general director Christopher Stokes strongly condemned the actions of the US forces.

"The frightening catalogue of errors outlined today illustrates gross negligence on the part of US forces and violations of the rules of war," Stokes said.

Campbell described how a Special Operations AC-130 gunship aircraft hit the hospital instead of a nearby Afghan intelligence compound that was thought to have been commandeered by Taliban fighters during their brief capture of the northern provincial capital.

Those who requested and executed the strike "did not undertake appropriate measures to verify that the facility was a legitimate military target", he said.

Some of those involved in the attacks failed to follow the rules of engagement, said Brigadier General Wilson Shoffner at the same press conference.

Shoffner refused to say if the US probe would be followed by an additional independent international investigation, which MSF has repeatedly called for.

Stokes reiterated the charity's position, saying the investigation leaves "more questions than answers" and the attack "cannot only be dismissed as individual human error or breaches of the US rules of engagement".

- 29 minutes -

The strike began at 2:08 am local time, Campbell said, and at 2:20 am MSF phoned the US military to report they were under attack.

"It took the headquarters and the US special operations commander until 2:37 am to realise the fatal mistake. At that time, the AC-130 had already ceased firing. The strike lasted for approximately 29 minutes," he added.

The gunship's electronic systems had malfunctioned, he said, cutting off much of its communications, and the aircraft had diverted from its path believing it had been targeted by a missile, degrading the accuracy of "certain target systems".

This meant that when the crew entered the coordinates they were directed to an open field some 300 metres (feet) from the intended target.

"The investigating officer found that the air crew visually located the closest, largest building near the open field which we now know was the MSF trauma centre," Campbell said.

The hospital appeared similar to the description provided of the intended target, and at night the crew was "unable to identify any signs of the hospital's protected status".

"We have learned from this terrible incident," said Campbell.

"We will also take administrative and disciplinary action," he said, adding that those involved would face "standard military justice".

US forces will offer their assistance to MSF in rebuilding the hospital, the only trauma centre in the region, he said.

- Contrasting accounts -

The timeline given by Campbell differed in parts from MSF's own account of the attack, which the charity has said lasted for around an hour.

In an internal review released earlier this month, MSF detailed frantic efforts by its staff to reach US and NATO officials to halt the strike.

According to that report, by 2.37 am -- when Campbell said US officials had realised the mistake -- four phone calls had been made by MSF representatives in Kabul and New York to NATO, the Red Cross, a UN military liaison, and the US defence department.

The calls continued after Campbell said the gunship had stopped firing, with one message at 2.56 am "insisting that the airstrikes stop and informing that we suspected heavy casualties", according to the MSF account.

In the days after the attack the US military offered a series of shifting explanations before President Barack Obama admitted in a call to MSF chief Joanne Liu that it had been a mistake and apologised.

A NATO statement released hours after the attack did not confirm the hospital was targeted, characterising it instead as "collateral damage" as Afghan forces came under fire.

The next day the US confirmed the hospital was hit directly but did not offer further details.

Later General Campbell suggested that Afghan forces had called in the strike, before offering a fourth account admitting US special forces had been in touch with the aircraft.

NATO and the Afghan army are conducting their own investigations.

burs-st/as/mfp