Doctors Without Borders (MSF) said at least nine of its staff members had been killed in the airstrike on Saturday that killed at least 16 people, including three children.

"The MSF trauma center in Kunduz was hit several times during sustained bombing and was very badly damaged," the group said in a statement released on Saturday.

"It is with deep sadness that we confirm, so far, the death of nine MSF staff during the bombing... 37 people were seriously wounded... There are many patients and staff who remain unaccounted for," the statement said, adding that the attack occurred at 2:10 a.m. local time (2040 UTC).

United Nations concerned about 'inexcusable' airstrike

UN human rights chief Zeid Ra'ad Al Hussein called the airstrike "tragic, inexcusable, and possibly even criminal." He also said that if a court were to establish the attack was deliberate, it could constitute a war crime.

"International and Afghan military planners have an obligation to respect and protect civilians at all times, and medical facilities and personnel are the object of a special protection," he said in a statement. "These obligations apply no matter whose air force is involved, and irrespective of the location."

US response

US Secretary of Defense Ash Carter said a "full investigation" was taking place over the deadly airstrikes.

"While we are still trying to determine exactly what happened, I want to extend my thoughts and prayers to everyone affected," he said in a statement.

Carter acknowledges that US forces were "operating nearby" the Doctors Without Borders hospital.

"A full investigation into the tragic incident is underway in coordination with the Afghan government. At this difficult moment, we will continue to work with our Afghan partners to try and end the ongoing violence in and around Kunduz."

'Thirty minutes of bombing'

MSF staff said the attack had gone on for some time after officials in Washington and Kabul had been informed and that both US and Afghan forces knew where the hospital was located.

"The bombing continued for more than 30 minutes after American and Afghan military officials in Kabul and Washington were first informed," MSF said, adding that "all parties to the conflict... were clearly informed of the precise location (GPS coordinates) of the MSF facilities."

Controversial air attacks

The incident could revive controversy about civilian casualties caused by US airstrikes in Afghanistan, an issue that caused a deep rift between Washington and former Afghan President Hamid Karzai.

The hospital is the only medical facility in the whole northeastern region of Afghanistan that has the capability to cope with serious injuries.

The bombing took place amid continued fighting in and around Kunduz. Government forces have been attempting to dislodge remaining Taliban insurgents after taking control of the city on Thursday.

Taliban fighters had seized the city on Monday in a major milestone in their 14-year-old insurgency.

The Afghan Defense Ministry expressed sadness but in a statement said "a group of terrorists armed with light and heavy weapon (...) were using the hospital building as a position to target Afghan forces and civilians."

The Afghan Ministry of Defense deputy spokesman, Brigadier General Dawlat Waziri, told The Associated Press news agency that helicopter gunships had fired on the militants, causing damage to the buildings. Interior Ministry spokesman Sediq Sediqqi said 10 to 15 "terrorists" had been hiding in the hospital at the time of the strike.

NATO unclear about nature of attack

NATO conceded US forces may have been behind the strike but has not so far commented on the specific claims of MSF, which has long treated the war-wounded from all sides of the conflict.

"US forces conducted an airstrike in Kunduz city at 2:15 am (local time)... against individuals threatening the force," a NATO statement said. "The strike may have resulted in collateral damage to a nearby medical facility. This incident is under investigation."

Taliban reaction

In a statement, the Taliban accused "barbaric American forces" of deliberately carrying out Saturday's strike, which "killed and wounded tens of doctors, nurses and patients."

Taliban spokesman Zabihullah Mujahid had earlier issued a statement saying that there were no Taliban fighters in the hospital at the time of the bombing.

'Undermined capacity'

The bombing of the hospital has been condemned by the International Committee of the Red Cross (ICRC), which said it was "deeply shocked."

"This is an appalling tragedy," said Jean-Nicolas Marti, who is the ICRC head in Afghanistan.

"Such attacks undermine the capacity of humanitarian organizations to assist the Afghan people at a time when they most urgently need it."

tj,ss/sms (Reuters, AP, AFP, dpa)