Asked whether he meant conducting explosive tests, Trump told reporters on Air Force One, including from AFP: "I am not going to say."

"You'll find out very soon, but we're going to do some testing, yeah, and other countries do it. If they're going to do it, we're going do it," he said on his way for a weekend at his golf resort in Florida.

No country other than North Korea has conducted an explosive nuclear weapon test for decades. Russia and China have not carried out such tests since the 1990s and the last US one was in 1992.

The 79-year-old Republican first made his surprise testing announcement by social media post on Thursday, minutes before entering a summit with Chinese leader Xi Jinping in South Korea.

The announcement came in the wake of Russia saying it had tested a nuclear-powered and nuclear-capable torpedo, as well as a new cruise missile.

Iran, which has been in conflict with the United States for years over its controversial nuclear projects, called Trump's directive "regressive and irresponsible."

"A nuclear-armed bully is resuming testing of atomic weapons. The same bully has been demonizing Iran's peaceful nuclear program," Foreign Minister Abbas Araghchi posted on social media.

Japanese atomic bomb survivors group Nihon Hidankyo sent a letter of protest to the US embassy in Japan.

The order "directly contradicts the efforts by nations around the world striving for a peaceful world without nuclear weapons and is utterly unacceptable," the Nobel Peace Prize-winning group said in the letter obtained by AFP.

- 'Pretty responsible' -

Amid expressions of concern abroad and among some in the US Congress, Defense Secretary Pete Hegseth said "resuming testing is a pretty responsible" policy.

Vice President JD Vance said the US nuclear arsenal needed to be tested to ensure it actually "functions properly." He too did not elaborate on what type of tests Trump had ordered.

But China's foreign ministry spokesman Guo Jiakun urged the United States to "earnestly abide" by a global nuclear testing ban.

United Nations chief Antonio Guterres said through a spokesman that "nuclear testing can never be permitted under any circumstances."

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

- Russia pushes back -

Russia pushed back against any idea that its recent weapon systems tests could justify a return to live test explosions.

The Kremlin questioned whether Trump was well-informed.

The recent weapons drills "cannot in any way be interpreted as a nuclear test," spokesman Dmitry Peskov told journalists.

"We hope that the information was conveyed correctly to President Trump."

Peskov then implied that Russia would conduct its own live detonation tests if Trump did it first.

According to the Stockholm International Peace Research Institute, the Russian stockpile of deployed and stored warheads is 4,309, compared to 3,700 for the United States and 600 for China -- figures that don't take into account hundreds of other warheads scheduled for dismantling.

- Last US test in 1992 -

The United States conducted 1,054 nuclear tests between July 16, 1945, when the first was conducted in New Mexico, and 1992.

Its two nuclear attacks on Japan during World War II make it the only country to have used the weapons in combat.

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

Then-president George H.W. Bush imposed a moratorium on further tests in October 1992 that has been continued by successive administrations.

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

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