Israel on Thursday formally banned the main United Nations aid agency for Palestinian refugees from operating on its territory despite heavy diplomatic backlash, a move that humanitarian officials warn could have a disastrous impact on aid delivery and jeopardize regional stability in the long term.

The order against the agency, known as UNRWA, went into effect at midnight on Thursday, the Israeli Foreign Ministry said. But hours later, there was no change on the ground except in the most cosmetic sense — right-wing activists clambered up the high stone walls of UNRWA's headquarters compound in east Jerusalem, yanked down the blue-and-white U.N. flag, hoisting up Israel's blue-and-white instead.