The international organization said in a statement issued at its New York headquarters that thousands of tons of donated supplies are still sitting in Egypt, Jordan and the ports of Israel.
"That aid should be right here, right now, helping the people who need it," said John Ging, director of operations in Gaza for the U.N. Relief and Works Agency for Palestine Refugees in the Near East.
While 120 truckloads of basic commodities make it through the crossings on good days, the agency said, the normal daily requirement is a minimum of 500.
Ging said the number of people receiving U.N. food aid in Gaza following Israel's three-week offensive against Hamas has reached 900,000.