Russia and China on Thursday vetoed a UN Security Council resolution calling for a ceasefire in Syria’s war-torn Idlib province after other members warned the conflict could become the century’s worst humanitarian crisis.

During negotiations on the draft, Russia pressed unsuccessfully to include an exception for “anti-terrorist operations.”

Ambassadors of Belgium, Kuwait and Germany had strongly urged that the resolution should not be vetoed, calling it a “purely humanitarian” measure.

The failed resolution said that all parties should cease hostilities “to avoid a further deterioration of the already catastrophic humanitarian situation in Idlib Governorate, beginning at noon Damascus time on 21 September.”

Russia and China countered by presenting an alternative resolution, which may be voted on later in Thursday’s session.

It specifies that the ceasefire would not apply to military operations against individuals, groups or entities “associated with terrorist groups.”

Russia backs the Syrian regime of Bashar al-Assad, which has been conducting a bloody four-month-long campaign to wrest Idlib province from jihadist forces.

On Wednesday, UN Secretary-General Antonio Guterres joined the head of the International Committee of the Red Cross Peter Maurer in appealing for an end to air and artillery bombardment of Idlib.