Update (1pm): An air strike on a medical facility in northern Syria has killed at least five staff members, the medical relief organisation that runs the facility said.
The Paris-based International Union of Medical Care and Relief Organisations, known by its French initials UOSSM, said the attack last night levelled a medical triage point it operates in rebel-held territory outside the contested city of Aleppo.
The US office of UOSSM said two nurses and two ambulance drivers were killed and one nurse remained in a critical condition following the attack on the medical facility in Khan Touman.
It said two of its ambulances, which are run by UOSSM and the World Health Organisation, were destroyed and the three-story building collapsed.
"This is a deplorable act against health care workers and medical facilities," said Dr Khaula Sawah, the head of UOSSM USA.
The Britain-based Syrian Observatory for Human Rights monitoring group said at least 13 people were killed in the attack, including nine militants, some of them belonging to the al Qaida-linked Fatah al-Sham Front.
There were no reports on who was behind the strike.
The medical facility attack follows a Monday night air strike on a Syrian Arab Red Crescent aid convoy that prompted international condemnation and recrimination 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 aid materials from the UN.
The incident exposed rising tensions between the two architects of Syria's ceasefire deal, Russia and the US.
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 operate an air force.
In New York on Tuesday, Russian and US. diplomats insisted that the Syrian ceasefire, which went into effect nine days ago, was not dead, despite indications of soaring violence.
The Syrian military declared on Monday night the truce had expired, shortly before presumed Russian or Syrian government jets launched a sustained aerial attack on Aleppo's opposition-held neighbourhoods.
The ceasefire was intended in part to allow humanitarian convoys to reach besieged and hard-to-reach areas throughout Syria.
Yet following the convoy attack, the UN. suspended overland aid operations to hard-to-reach areas in Syria. Syrians living in opposition areas will be disproportionately affected because the UN's major warehouses are located in government-held areas.
The UN estimates six million Syrians live in besieged and hard-to-reach areas.