U.S. President Donald Trump on Thursday ordered the U.S. military to immediately resume testing nuclear weapons, for the first time in 33 years, minutes before beginning a meeting with Chinese President Xi Jinping, writes Reuters.

Trump made the surprise announcement on Truth Social while he was aboard his Marine One helicopter flying to meet Xi for a trade negotiating session in Busan, South Korea. He said he was instructing the Pentagon to test the U.S. nuclear arsenal on an "equal basis" with other nuclear powers.

"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.

"Russia is second, and China is a distant third, but will be even within 5 years".

He did not elaborate and did not reply to a reporter`s shouted question about his post after his initial remarks to Xi. It was not immediately clear whether Trump was referring to nuclear-explosive testing, which would be carried out by the National Nuclear Safety Administration, or flight testing of nuclear-capable missiles.