At least 14 people were killed by airstrikes apparently targeting a medical facility in southern Aleppo two days after a fierce attack on an aid convoy in the same city left 21 dead, a medical organization told the DPA news agency Wednesday.

The clinic in the village of Khan Tuman was completely leveled in the 11:00 p.m. (2000 UTC) strike and more people are feared to be trapped under rubble, the group added.

It's unclear who is responsible for the airstrike. Russia and Syrian warplanes, as well as a US-led aerial coalition, are all operating in Syrian airspace targeting suspected "Islamic State" fighters as well as anti-regime rebels.

"We don't yet know exactly how many dead there are," Ahmed Dbais, trauma director of Union of Syrian Medical Relief Organizations (UOSSM), a France-based coalition of relief workers founded by Syrian expatriates, said in a statement.

"The building has three floors, including a basement. Because of the intensity of the bombardment, the three stories collapsed and are completely destroyed," the statement continued.

The head of UOSSM France, Dr Ziad Alissa, condemned the "unacceptable" attack on the group's clinic and staff.

"Deliberately targeting humanitarian workers and medical professionals is a clear violation of international humanitarian law," he said. "We appeal to the international community to act swiftly to put a stop to these atrocities. Too many lives have been lost."

The US says it holds Russia responsible for airstrikes by either Syria or Russia that hit an aid convoy in this week. It's unclear who was responsible for the latest attack on a clinic.

Khan Tuman is not far from Orum al-Kubrah, the site of an attack on an aid convoy and warehouses that killed at least 20 civilians on Monday. That's triggered a war of words between the US and Russia with Moscow denying responsibility for the strike as hopes for reviving a ceasefire diminish with each day.

Washington has been holding out hope that the UN could negotiate a new truce deal with the Russians. The week-long truce did manage to reduce the violence in the country and allowed some aid to reach tens of thousands of people in need.

But Kremlin spokesman Dmitry Peskov said Russia did not expect to reinstate the ceasefire, according to state news agency TASS. And the UN on Tuesday halted all aid convoys until further notice in response to Monday's attack.

jar/msh (AFP, dpa)