An airstrike on a medical site near Aleppo, Syria, left at least four aid workers dead overnight on Tuesday, NBC News reported.
The attack in the town of Khan Touman, controlled by rebels, comes amid a fragile cease-fire brokered by the U.S. and Russia. But another airstrike hit a convoy of aide workers Monday, killing at least 20 people and touching off a round of blame-trading.
The Associated Press reports that both nations claim the others' aircraft were overhead when the convoy was struck, and it was not immediately clear who launched the airstrike that hit the medical facility overnight on Tuesday.
The humanitarians killed in Tuesday's strike were ambulance drivers and nurses who worked for the Union of Medical Care and Relief Organizations. A fifth person injured, a nurse, was critically injured in the strike.
The organization condemned the "unacceptable" loss of life, and said the medical center was completely destroyed and left victims buried under debris.