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 social media 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."

The International Campaign to Abolish Nuclear Weapons (ICAN) says nine countries possess nuclear weapons: Russia, the United States, China, France, United Kingdom, Pakistan, India, Israel and North Korea.

Of the roughly 12,331 nuclear warheads ICAN counts, more than 5,500 belong to Russia while the United States owns 5,044.

Trump offered no details of the precise nature of the testing to be undertaken, but said the process would "begin immediately."

Putin announced on Wednesday the successful testing of a nuclear-capable, nuclear-powered underwater drone, the second weapons test in days.

In televised remarks broadcast from a military hospital treating Russian soldiers wounded in Ukraine, Putin said there was "no way to intercept" the unmanned drone torpedo dubbed "Poseidon."

He said Poseidon can travel faster than conventional submarines, dive deep and reach any continent in the world.

After a first test of a cruise missile on Sunday, Trump chided Putin saying he ought to end the war in Ukraine "instead of testing missiles."

Last week, a planned summit between Trump and Putin in Budapest was scrapped.

Between 1945 -- with the first ever atomic bomb test in New Mexico on July 16 -- and 1992, the United States has conducted 1,054 nuclear tests and carried out two nuclear attacks on Japan during World War II.

The last US nuclear test explosion was in September 1992, with a 20-kiloton underground detonation at the Nevada Nuclear Security Site.

In October 1992, then-president George H.W. Bush imposed a moratorium on further tests, which was continued by successive administrations. Nuclear testing was replaced by non-nuclear and subcritical experiments using advanced computer simulations.

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.