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.

Since 1996 the United States has been a signatory to the the Comprehensive Nuclear-Test-Ban Treaty, which bans all atomic test blasts, whether for military or civilian purposes

It was not immediately clear if Trump was referring to testing nuclear warheads -- something the United States last did in 1992 -- or testing weapons systems capable of carrying atomic warheads.

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

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

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.

In a press huddle aboard Air Force One, Trump noted that it had been "many years" since the United States had conducted nuclear tests.

"We don't do testing... we've halted it years, many years ago," adding that to start again was "appropriate" because others are testing.

"I'd like to see denuclearization... denuclearization, would be a tremendous thing," he added.

He claimed "it's something we are actually talking to Russia about, and China would be added to that if we do something."

Trump kept the location and dates for testing vague during the press conference, but earlier said it would "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.

China later urged the US to "earnestly abide" by a global nuclear testing ban.

"China hopes the United States will earnestly abide by the obligations of the comprehensive nuclear-test-ban treaty and its commitment to a ban on nuclear testing, and take concrete actions to safeguard the global nuclear disarmament," foreign ministry spokesman Guo Jiakun said.

Between 1945, when the first ever atomic bomb test happened in New Mexico on July 16, and 1992 the United States has conducted 1,054 nuclear tests and 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 has been continued by successive administrations.

Nuclear testing was replaced by non-nuclear and subcritical experiments using advanced computer simulations.

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.