At least four aid workers were killed in an overnight airstrike on a medical triage point near Aleppo, according to their charity.
The attack in the rebel-held town of Khan Touman came one day after airstrikes hit an aid convoy in the nearby town of Uram al-Kubra, killing at least 20 people and touching off a round of blame-trading.
The U.S. said Russia was responsible for the airstrike on the aid convoy; Moscow vehemently denied any involvement.
It was not immediately clear who launched the airstrike that hit the medical facility overnight on Tuesday.
The slain humanitarians worked for the Union of Medical Care and Relief Organizations and were rushing to pick up wounded from a separate attack when their ambulances were hit. Two nurses and two ambulance drivers were killed, according to the organization.
It said a fifth staff member — a nurse — was critically injured in the airstrike, which "completely destroyed" the medical center and left "many victims" buried in the rubble.
The organization condemned the "unacceptable" loss of life, saying in a statement that aid workers and medical personnel are protected under international law.
"They give their lives to save others," the organization added. "The instigators of these acts must be held responsible."
It declined to comment on who may have conducted the attack, saying only that it was an "aerial bombardment."
According to the Britain-based Syrian Observatory for Human Rights, at least nine rebels also died in the airstrikes — with the death toll likely to rise.