President Donald Trump reaffirmed on Friday that the U.S. would resume nuclear testing, and he did not answer directly when asked whether that would include the traditional underground nuclear tests that were common during the Cold War.

"You'll find out very soon, but we're going to do some testing" Trump told reporters aboard Air Force One as he flew to Palm Beach, Florida, when asked about underground nuclear tests.

"Other countries do it. If they're doing to do it, we're going to do it, OK?"

Trump on Thursday said he 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 that 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.

It was not immediately clear whether Trump was referring to nuclear-explosive testing, which would be carried out by the National Nuclear Security Administration, or flight testing of nuclear-capable missiles.

No nuclear power — other than North Korea most recently in 2017 — has carried out explosive nuclear testing in over 25 years.