The US airstrikes that destroyed a hospital belonging to Doctors Without Borders in the northern Afghan city of Kunduz was "a deliberate attack", a top official charged on Thursday.

"The trauma hospital came under multiple, precise and sustained air raids for more than an hour after we informed military officials in both Washington and Kabul," Christopher Stokes, the general director of the aid group, said in arguing that the attack did not consist only of "collateral damage".

"This does not match at all what our team told us," he said. "They said very precise strikes on the main building of the hospital. It was a deliberate hit."

He called for an independent inquiry into Saturday's airstrikes, which killed at least 22 people. The aid group said nine patients and 24 staff members are still missing.

Three investigations are now under way into the bombing by US, Nato and Afghan forces.

US President Barack Obama on Wednesday called the president of Doctors Without Borders to apologize for the attack.

"When we make a mistake we own up to it," White House spokesperson Josh Earnest said. "We apologise, and we make changes to be sure it won't occur again in the future."

Stokes, however, charged that the airstrikes amounted to violations of international humanitarian law and "an attack on the Geneva Conventions itself".

He pressed for an independent international investigation by the International Humanitarian Fact-Finding Commission, an international body that investigates potential breaches of international humanitarian law.

The commission was created in 1991 on the basis of the Geneva Conventions, but the commission has never been activated or used.

The 76 signatory states includes Germany, Russia and Britain, but the US and France are not part of it.

None of the signatory states has replied to the Doctors Without Borders request for a formal investigations yet, Stokes said.

DPA/News24.com