Russian President Vladimir Putin ordered his top officials on Wednesday to draft proposals for a possible test of nuclear weapons, something Moscow has not done since the 1991 collapse of the Soviet Union.
The order - responding to President Donald Trump's announcement last week that the US would resume testing - was a further signal that the two countries with the world’s largest nuclear arsenals are rapidly nearing a step that could sharply escalate geopolitical tensions.
"I am instructing the Foreign Ministry, the Defence Ministry...the special services and relevant civilian agencies to do everything possible to collect additional information on the issue, analyse it at the Security Council and make agreed proposals on the possible start of work on the preparation of nuclear weapons tests," Putin said in televised remarks.
Russia-US relations have deteriorated sharply in the past few weeks as Trump, frustrated with a lack of progress towards ending the war in Ukraine, has cancelled a planned summit with Putin and imposed sanctions on Russia for the first time since returning to the White House in January.
Putin issued his instruction at a meeting of his Security Council, where parliamentary speaker Vyacheslav Volodin departed from the official agenda of transport safety to ask how Moscow would respond to Trump’s plans to carry out US nuclear testing for the first time in 33 years.
The question, though meant to appear spontaneous, triggered a series of clearly prepared interventions.
Defence Minister Andrei Belousov told Putin that recent US remarks and actions meant it was "advisable to prepare for full-scale nuclear tests" immediately.
Russia’s Arctic testing site at Novaya Zemlya could host such tests at short notice, Belousov added.
General Valery Gerasimov, head of the General Staff, told Putin: "If we do not take appropriate measures now, time and opportunities for a timely response to the actions of the United States will be lost, since the time required to prepare for nuclear tests, depending on their type, ranges from several months to several years."
No country apart from North Korea - most recently in 2017 - has carried out explosive tests of nuclear weapons in the 21st century. Security analysts say a resumption of testing by any of the world’s nuclear powers would be destabilising, as it would likely trigger a similar response by the others.
"Action-reaction cycle at its best. No one needs this, but we might get there regardless," Andrey Baklitskiy, senior researcher at the UN Institute for Disarmament Research, posted on social media.
Russia and the US are by far the biggest nuclear powers by numbers of warheads, followed by China, France, Britain, India, Pakistan, Israel and North Korea, according to the Federation of American Scientists.
State news agency TASS quoted Kremlin spokesperson Dmitry Peskov as saying Putin had set no specific deadline for officials to draft the requested proposals.
"In order to come to a conclusion about the advisability of beginning preparations for such tests, it will take exactly as much time as it takes for us to fully understand the intentions of the United States of America," Peskov said.
Trump has yet to clarify whether the resumption he ordered last week referred to nuclear-explosive testing or to flight testing of nuclear-capable missiles.
Russia last month tested its new Burevestnik cruise missile, which is nuclear-powered and designed to carry a nuclear warhead. It also held nuclear launch drills and tested a nuclear-powered Poseidon super-torpedo.
Testing delivery systems for nuclear weapons does not involve a nuclear explosion. Such blasts were regularly staged by the nuclear powers for decades during the Cold War, with devastating environmental consequences that campaigners fear could be unleashed once again if explosive tests resume.
(Reporting by Vladimir Soldatkin in Moscow and Mark Trevelyan in London; Additional reporting by Maxim Rodionov, Guy Faulconbridge and Lucy Papachristou Editing by Gareth Jones)