The U.N. humanitarian chief says his worst fears that a full-scale military onslaught in northwestern Syria would "unleash a humanitarian nightmare unlike any we have seen" in Syria are now coming true.

Mark Lowcock told a Security Council meeting Friday that escalating violence in the last three weeks in the de-escalation zone in rebel-held Idlib, where about 3 million people live, killed up to 160 people, displaced at least 180,000 and left millions "crammed into an ever smaller area."

U.N. political chief Rosemary DiCarlo warned that "if the escalation continues and the offensive pushes forward, we risk catastrophic humanitarian fallout and threats to international peace and security."

She reiterated the U.N.'s call for an urgent de-escalation of violence and urged Turkey and Russia to re-establish a cease-fire in northwest Syria.