U.S. President Donald Trump on Thursday ordered the U.S. military to immediately restart the process for testing nuclear weapons after a halt of 33 years, a move that appeared to be a message to rival nuclear powers China and Russia.

Trump made the surprise announcement on Truth Social while aboard his Marine One helicopter flying to meet Chinese President Xi Jinping for a trade-negotiating session in Busan, South Korea.

"Because of other countries testing programs, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis. That process will begin immediately," Trump posted.