Shortly before meeting with Chinese President Xi Jinping in South Korea, US President Donald Trump posted on social media that he had instructed the Pentagon to start "immediately" testing nuclear weapons.
The reason, he said, was because other countries "seem to all be nuclear testing".
"We have more nuclear weapons than anybody. We don't do testing. I see them testing and I say, well, if they're going to test, I guess we have to test," he later told reporters aboard Air Force One.
The US has not conducted nuclear weapons tests detonating warheads for 33 years.
Mr Trump has not specified what type of testing he had in mind, apart from being "on equal basis" with other countries.
If the US resumes explosive nuclear weapons tests, experts warn it will be a dangerous escalation, especially at a time when nuclear threats were on the rise.
How nuclear weapons are tested
The thought of nuclear testing brings up images of mushroom clouds billowing over the skies in remote locations.
Before the 1963 Partial Test Ban Treaty, this type of atmospheric testing was regularly carried out by several countries.
Underwater testing was also taking place, where nuclear explosions close to the surface created large columns of water and steam that would shoot into the air.
When these practices were banned, the testing went substantially underground.
Nobel Peace Prize laureate Professor Tilman Ruff, founding member of the International Campaign to Abolish Nuclear Weapons (ICAN), said more than 2,000 nuclear explosions were detonated worldwide between 1945 and 1996.
Those tests "dispersed radioactivity uncontrollably all over the world", he told the ABC.
Dr Ruff, who is an infectious diseases physician and professor at Melbourne University's school of population and global health, said the fallout continued to have profound impacts.
"All of us bear in every cell of our bodies fallout from past nuclear weapons tests," he said.
"And subsequent generations will for many thousands of linear hence. There will be, in total, several million excess cancer deaths from that fallout."
Other than North Korea, no country has tested nuclear weapons since the late 1990s.
The US military regularly tests its missiles that are capable of delivering a nuclear warhead, but it has not detonated the weapons since 1992.
"The US is constantly conducting missile defence tests … So in a sense, that's nothing new," Dr Ruff said.
"What would be new and a very significant and regrettable escalation, and a backward step, would be if he's talking about nuclear weapons explosive testing."
US underground test sites
It is unclear if Mr Trump is talking about testing nuclear weapons delivery systems — such as launching a nuclear-capable missile — or explosive nuclear testing.
When asked where tests would take place, he said, "It'll be announced. We have test sites".
The US has extensive facilities in Nevada, about 96 kilometres north-west of Las Vegas, which were previously used for underground tests.
The sites have not been used for decades, but Dr Ruff said it was "disturbing" that they still existed.
"Russia, China and the US have made some efforts to keep those sites in readiness and not just decommission them as France has done, for example," he said.
But if the US were to use the Nevada facilities for underground tests, it would take a considerable amount of time to reactivate, modernise and recommission the equipment, he added.
Even if nuclear tests are conducted underground, they still carry risks, Margaret Beavis, co-chair of ICAN, said.
If underground nuclear tests "vent" to the surface, they can produce considerable radioactive debris and potential leakage into groundwater.
"Contaminated, radioactive contaminated water is pretty problematic," Dr Beavis said.
The last known underground test was carried out by North Korea on September 3, 2017.
On that date, the US Geological Survey recorded a magnitude-6.3 earthquake near Sungjibaegam, which is home to North Korea's nuclear test site.
Dr Beavis said explosive nuclear tests had largely stopped because advances made it possible to test the weapons without having to explode them.
"There were tests using high-powered computers and nuclear physics experiments," she told the ABC.
"And then some critical testing, which is where you test the mechanism, but it doesn't actually go onto a full explosion."
Trump's false claims
Vice President JD Vance said that the US nuclear arsenal needed to be tested to ensure it actually "functions properly", but did not elaborate on what type of tests Mr Trump had ordered.
The president's statement "speaks for itself", Mr Vance told reporters at the White House.
In April, Brandon Williams, the under secretary for the US Nuclear Security at the Department of Energy, said the country's "deployed nuclear stockpile remains safe, secure, and effective without nuclear explosive testing".
Heather Williams, senior fellow in the defence and security department at the US Center for Strategic and International Studies, said the US was able to rely on "high-energy lasers and supercomputing" to test its nuclear arsenal.
In his social media post, Mr Trump claimed that the US had more nuclear weapons "than any other country" and accused other states of carrying out tests.
The United States has a stockpile of 5,225 nuclear warheads and Russia has 5,580, according to the Washington-based Arms Control Association, a nonpartisan group advocating nuclear nonproliferation agreements.
The Pentagon estimates that China will have more than 1,000 nuclear weapons by 2030.
Dr Ruff said the post contained a number of factual errors.
"The US doesn't have the largest nuclear arsenal," he said.
"And nobody is conducting nuclear explosive testing at the moment, as implied."
The Comprehensive Nuclear Test Ban Treaty, which the US signed but did not ratify, has been observed since its adoption by all countries possessing nuclear weapons; North Korea is the only exception.
Russia pushes back on test claims
The US president's comments came days after Russia declared it had tested nuclear-capable, nuclear-powered cruise missiles and sea drones.
The recent weapons drills "cannot in any way be interpreted as a nuclear test", Kremlin spokesperson Dmitry Peskov told journalists.
"We hope that the information was conveyed correctly to President Trump."
Russian President Vladimir Putin has repeatedly said that if any country tests a nuclear weapon, then Russia would do so too.
While testing nuclear-powered weapons is not the same as testing nuclear weapons themselves, Russia's tests were provocative, Stephan Fruehling from the ANU's Strategic and Defence Studies Centre, said.
Professor Fruehling does not believe that Mr Trump's comments indicate a change in America's nuclear posture.
"I think that this is sending a signal to the Russians that don't think that by playing on nuclear fears that's going to be effective … in the end, America can do the same," he said.
Nuclear proliferation on the rise
According to the Stockholm International Peace Research Institute (SIPRI), nine countries possess nuclear weapons: .
The institute has observed that, in recent years, many of those nuclear arsenals have been enlarged and upgraded.
Dr Ruff warned that Mr Trump's calls came at an "extraordinarily dangerous" time in history.
"The number of nuclear weapons are going up again, particularly the militarily deployed and high alert weapons," he said.
"We're in a pretty bad place in terms of nuclear risks; almost everything is going in the wrong direction."
Nearly all of the hard-won treaties that constrained nuclear weapons since the end of the Cold War have been abrogated, he added.
The last remaining nuclear arms treaty constraining 90 per cent of the world’s nuclear weapons is due to expire in February next year.
The only "good news" was that half of the world's nations had signed the UN Treaty on the Prohibition of Nuclear Weapons, Dr Ruff said.
The historic treaty provides the only internationally agreed framework for their eventual elimination and has now been signed by half of the world's nations.
But some countries that possess nuclear weapons, and others that seek protection from them — such as Australia — are yet to sign.
"The only way you can respond to nuclear war is to stop these weapons from being in existence. And the only way to do that is to increase pressure on the countries to disarm," Dr Beavis said.
"It is essential that Australia fast-tracks the signature of the UN Treaty on the Prohibition of Nuclear Weapons, to indicate we do not support nuclear weapons under any circumstances."
