The U.N. humanitarian chief is urging all combatants in Aleppo to agree to a 48-hour pause to deliver desperately needed aid, warning of a "humanitarian catastrophe unparalleled in the over five years of bloodshed" in Syria.

Stephen O'Brien says Aleppo is being bombed every day, including a dozen new attacks Monday, and has become "the apex of horror" in "the greatest crisis of our time."

He told the U.N. Security Council, which has been deeply divided over Syria, that "you have the power with a pen — a simple pen stroke — to allow food to people."

O'Brien says the U.N. sought to deliver aid to nearly one million Syrians in August but not a single humanitarian convoy has moved so far this month.