The deputy U.N. humanitarian chief is warning that further military operations in northwest Syria's Idlib province will overwhelm aid efforts, stressing that an estimated 3 million people are caught in crossfire in the last rebel stronghold.

Ursula Mueller told the Security Council on Tuesday that humanitarian operations in many areas where there are active hostilities have been suspended.

Mueller said that includes the suspension of health, nutrition and protection services previously supporting some 600,000 people, including 21 immunization centers that have ceased operations. She says it also includes at least 49 health facilities that have suspended or partially suspended activities for security reasons.

Mueller says that so far this month heavy shelling and aerial bombing has resulted in more than 160 civilian deaths and the displacement of some 270,000 people.