President Donald Trump said he would pull out of the World Trade Organization if it doesn’t treat the U.S. better, continuing his criticism of a cornerstone of the international trading system.

“If they don’t shape up, I would withdraw from the WTO,” Trump said Thursday in an interview with Bloomberg News at the White House.

A U.S. withdrawal from the WTO would severely undermine the post-World War II multilateral trading system that the U.S. helped build

Trump said last month that the U.S. is at a big disadvantage from being treated “very badly” by the WTO for many years and that the Geneva-based body needs to “change their ways.”

U.S. Trade Representative Robert Lighthizer has said allowing China into the WTO in 2001 was a mistake. He has long called for the U.S. to take a more aggressive approach to the WTO, arguing that it was incapable of dealing with a non-market economy such as China.

Lighthizer has accused the WTO dispute-settlement system of interfering with U.S. sovereignty, particularly on anti-dumping cases. The U.S. has been blocking the appointment of judges to the WTO’s appeals body, raising the possibility that it could cease to function in the coming years.