The United Nations says it cannot deliver aid to 600,000 people in two Syrian cities controlled by the Islamic State group.

Deputy humanitarian chief Kyung-wha Kang told the U.N. Security Council on Wednesday that residents of the Islamic State group's de facto capital of Raqqa and the city of Deir ez Zor received no food deliveries in December because the U.N. had no agreement with armed groups there.

The U.N.'s monthly reports on efforts to get food and other aid to millions trapped in Syria's civil war have been unrelentingly grim. Kang says 12 million people inside Syria need humanitarian aid, and 3.8 million people have fled to neighboring countries.

She urged the Security Council to find a way to end the war.