The move comes after Russian President Vladimir Putin said Wednesday that Moscow had successfully tested a nuclear-capable, nuclear-powered underwater drone, in defiance of Washington's warnings.

"Because of other countries testing programs, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis," Trump wrote in a Truth Social post that specifically referenced Russia and China.

Trump also noted the United States has more nuclear weapons than any other country, praising his own efforts to do "a complete update and renovation of existing weapons."

He added that "Russia is second, and China is a distant third, but will be even within five years."

Trump did not provide further details on the testing except that the "process will begin immediately."

Trump is in South Korea to meet with Xi, as the leaders of the world's top two economies come face-to-face for the first time in the Republican president's second term.