The U.N. humanitarian chief has called for immediate access to deliver "life-saving assistance" to 50,000 Syrians stranded on the sealed border with Jordan, amid reports of shrinking food supplies.

Mark Lowcock told the U.N. Security Council from Jordan late Monday that "access to food is precarious" for displaced Syrians in the desert camp of Rukban.

Aid officials say commercial food shipments to Rukban dropped sharply since the summer, after Syrian government forces advanced in the area.

They say black market prices have soared and that moderate, acute malnutrition among young children is on the rise.

U.N. food deliveries from Jordan have been sporadic. Jordan balks at regular aid shipments from its soil, citing security concerns.

Lowcock says U.N. agencies "are straining every sinew" to try to deliver aid from Syria.