Afghan staff react inside a Medecins Sans Frontieres (MSF) hospital after an air strike in the city of Kunduz, Afghanistan in this October 3, 2015 MSF handout photo. The US military on Saturday acknowledged it may have bombed a hospital run by medical aid group Medecins Sans Frontieres in the Afghan city of Kunduz in an air strike that killed at 22 people and wounded 37. [Photo/Agencies/Medecins Sans Frontieres]

KABUL - Medical aid group Medecins Sans Frontieres (MSF) on Sunday demanded an independent international inquiry into a suspected US air strike that killed 22 people in an Afghan hospital it runs, branding the attack a "war crime".

MSF said a US military probe into the incident, which occurred during a push by Afghan security forces to retake the key northern city of Kunduz from Taliban insurgents, was not enough.

"Relying only on an internal investigation by a party to the conflict would be wholly insufficient," MSF General Director Christopher Stokes said in a statement.

"Under the clear presumption that a war crime has been committed, MSF demands that a full and transparent investigation into the event be conducted by an independent international body," Stokes said.

Battles were still raging on Sunday around Kunduz, a city of 300,000, as government forces backed by US air power sought to drive out the Taliban militants who seized the city almost a week ago in one of their biggest victories in the 14-year war.

Decomposing bodies littered the streets and trapped residents said that food was running scarce.

Any confirmation of US responsibility for the hospital deaths would deal a blow to Afghan President Ashraf Ghani's policy of forging closer ties with the United States. His predecessor Hamid Karzai fell out with his backers in Washington in part over the number of civilians killed by US strikes.

But the Afghan leader will be torn between distancing himself from Washington and the need for American firepower to help his forces drive insurgents out of Kunduz.

The US military said it conducted an air strike "in the vicinity" of the MSF hospital as it targeted Taliban insurgents who were directly firing on US military personnel. It has not acknowledged hitting the hospital.

President Barack Obama offered condolences to the victims of what he called "the tragic incident". The U.N. human rights chief said the hospital assault was "inexcusable" and also said it could amount to a war crime.

The US-led coalition force in Afghanistan said it expected to complete its preliminary multi-national investigation within days.

In Kabul, the Ministry of Defence said Taliban fighters had attacked the hospital and were using the building "as a human shield". But MSF denied this.

"Not a single member of our staff reported any fighting inside the MSF hospital compound prior to the US airstrike on Saturday morning," Stokes said.