United States President Donald Trump has landed back in the US after a surprise directive to begin nuclear weapons testing that has raised the spectre of renewed superpower tensions.
Trump announced the order on social media, just as he was entering a summit with Chinese leader Xi Jinping in South Korea on Thursday.
It came days after Russia declared it had tested nuclear-capable, nuclear-powered cruise missiles and sea drones.
The blunt statement from Trump, who boasts frequently about being a "peace" president, left much unanswered.
Chiefly, it was unclear whether he meant testing weapons systems or actually conducting test explosions — something the US has not done since 1992.
“Because of other countries' testing programs, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis," Trump said in a post on his Truth Social platform.
Trump also said that the US has more nuclear weapons than any other country and that he had achieved this in his first term as president.
The Stockholm International Peace Research Institute said in its latest annual report that Russia possesses 5,489 nuclear warheads, compared to 5,177 for the United States and 600 for China.
In his post, Trump said — minutes ahead of his meeting with Xi — that China was expected to "be even within 5 years", without substantiating the claim.
China, Russia express concerns
In response to Trump's announcement, Chinese foreign ministry spokesperson Guo Jiakun urged the US to "earnestly abide" by a global nuclear testing ban.
Russia questioned whether Trump was well-informed about its activities.
"President Trump mentioned in his statement that other countries are engaged in testing nuclear weapons. Until now, we didn't know that anyone was testing," Kremlin spokesman Dmitry Peskov told reporters.
Russia's recent weapons drills "cannot in any way be interpreted as a nuclear test", Peskov said. "We hope that the information was conveyed correctly to President Trump."
Peskov then implied that Russia would conduct its own live warhead tests if Trump did it first.
“If someone departs from the moratorium, Russia will act accordingly,” Peskov said.
Russian President Vladimir Putin has repeatedly said that if any country tests a nuclear weapon, then Russia will do so too.
Both countries observe a de facto moratorium on testing nuclear warheads, though Russia and the United States do regularly run military drills involving nuclear-capable systems.
The US 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.
United Nations secretary-general António Guterres said through his deputy spokesman that "nuclear testing can never be permitted under any circumstances".
Trump also repeated in his remarks to reporters a previous claim that he wants negotiations on reducing nuclear weapons forces.
"Denuclearisation would be a tremendous thing," he said. "It's something we are actually talking to Russia about, and China would be added to that if we do something."
Last US test in 1992
The US conducted 1,054 nuclear tests between July 16, 1945, when the first test was conducted in New Mexico, and 1992, as well as two nuclear attacks on Japan during World War Two.
It is the only country to have used nuclear weapons in combat.
The last US nuclear test explosion was in September 1992, with a 20-kiloton underground detonation at the Nevada Nuclear Security Site.
Then-president George HW 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.
No nuclear power — other than North Korea most recently in 2017 — has carried out explosive nuclear testing in over 25 years.
Nevada congresswoman Dina Titus responded that she would introduce legislation to "put a stop" to any move to restore live weapons testing in her state.
US senator Jacky Rosen, also a Nevada Democrat, said in a post on X that Trump's statement "directly contradicts the commitments I secured from Trump nominees ... who've told me explosive nuclear testing would not happen & is unnecessary".
