The International Union of Medical Care and Relief Organisations said the attack in northern Syria levelled a medical triage point in rebel-held territory outside the contested city of Aleppo.
The Britain-based Syrian Observatory for Human Rights put the toll at 13 dead – four nurses and paramedics and nine rebel fighters, some of them belonging to the al Qaida-linked Fatah al-Sham Front.
The monitoring group said the triage point was in the rebel-held town of Khan Touman, south of Aleppo.
The new air strike came as the US blamed Russia for an attack on an aid convoy that killed 20 civilians.
The United Nations announced it was suspending overland deliveries in Syria in response, jeopardising food and medical security for millions of besieged and hard-to-reach civilians.
Confusion continued about who struck the convoy, but the White House insisted it was either Russia or Syria.
White House deputy national security adviser Ben Rhodes said either way the US held Russia responsible because it was Russia’s job under a week-old ceasefire to prevent Syria’s air force from striking in areas where humanitarian aid was being transported.
“All of our information indicates clearly that this was an air strike,” he said, rejecting the claim by Russia’s Defence Ministry that a cargo fire caused the damage. Russia and Syria have denied carrying out the bombing.
Within a minute of the strike, the US tracked a Russian-made Su-24 warplane directly over the region of the attack, officials said. Both the Russian and Syrian air forces fly the Su-24, but US officials said there were strong indications that the jet was flown by the Russian military.
Witnesses described Monday’s attack on a Syrian Arab Red Crescent warehouse and convoy in the rebel-held town of Uram al-Kubra in Aleppo province as prolonged and intense, saying the aerial bombardment continued as rescue workers rushed to pull the wounded from the flaming wreckage and rubble.
The convoy was part of a routine dispatch operated by the Syrian Red Crescent, which UN officials said was delivering assistance to 78,000 people in Uram al-Kubra, west of Aleppo city. It was carrying food, medicines, emergency health kits, IV fluids, and other essentials supplied by the UN and the World Health Organisation.