The team had just arrived at the scene of the first air strike in the rebel-held town of Khan Touman when planes circled around and struck the area again, D Oubaida Al Moufti, vice-president of the International Union of Medical Care and Relief Organisations, said.
Syrian government forces have been accused of carrying out “double tap” attacks throughout the five-and-a-half-year war, placing paramedics and rescue workers in peril.
The organisation, known by its French initials UOSSM, had initially said that the Tuesday night strike levelled a medical triage point it operates in rebel-held territory outside the contested city of Aleppo.
The Britain-based Syrian Observatory for Human Rights said at least 13 people were killed in the attack, including nine militants, some of them belonging to the Fatah al-Sham Front, an al Qaida-linked group previously known as the Nusra Front.
Three nurses and two ambulance drivers died of their injuries, UOSSM said.
It was not immediately clear who carried out the strike. Aircraft from Syria, Russia and the US-led coalition are targeting the Fatah al-Sham Front, which along with the Islamic State group was excluded from a week-long ceasefire.
An air strike on a Syrian Arab Red Crescent aid convoy on Monday night prompted international condemnation over attacks targeting humanitarian facilities and workers.
UN secretary-general Ban Ki-moon described the convoy strike as a “sickening, savage and apparently deliberate attack”. The convoy was carrying UN aid.
The incident fuelled tensions between the two architects of Syria’s ceasefire, Russia and the United States.
The US said it believed Russian or Syrian government jets were behind the attack that killed 20 civilians, and that either way it held Russia responsible because under the truce deal Moscow was charged with preventing air strikes on humanitarian deliveries. Syria’s rebels do not have aircraft.
US Secretary of State John Kerry called for aircraft over humanitarian routes in northern Syria grounded in order to facilitate aid deliveries.