Donald Trump's intention to scrap a landmark nuclear missile treaty with Russia is a reversal of efforts to achieve nuclear disarmament, former Soviet leader Mikhail Gorbachev has said, while the Kremlin has branded the decision a "very dangerous step".
Meanwhile, a former UK ambassador to Moscow has warned that the scrapping of the Intermediate-Range Nuclear Forces (INF) Treaty could lead to a nuclear arms race between the US and Russia, as well as the proliferation of nuclear arsenals worldwide.
What has happened and what is President Trump threatening to withdraw from?
The warning from Moscow comes after the US President said he would pull America out of the INF Treaty - a Cold War era agreement signed by Mr Gorbachev and former US President Ronald Reagan, that prevents both countries from producing certain intermediate-range missiles - because Russia had violated the terms.
However, Mr Trump did not provide any details on these violations.
The 1987 pact helps protect the security of the US and its allies in Europe and the Far East, and prohibits the US and Russia from possessing, producing or test-flying a ground-launched cruise missile with a range of 300 to 3,400 miles.
What exactly did President Trump say?
“Russia has violated the agreement. They have been violating it for many years,” Mr Trump said after a rally in Elko, Nevada, on Saturday.
“We’re not going to let them violate a nuclear agreement and go out and do weapons and we’re not allowed to.”
The agreement has constrained the US from developing new weapons, but America will begin developing them unless Russia and China agree not to possess or develop the weapons, Mr Trump said.
However, China has never been part of the pact.
“We’ll have to develop those weapons, unless Russia comes to us and China comes to us and they all come to us and say let’s really get smart and let’s none of us develop those weapons, but if Russia’s doing it and if China’s doing it, and we’re adhering to the agreement, that’s unacceptable,” he said.
What else has Russia said?
Mr Gorbachev - who signed the 1987 treaty - said "Washington's desire to turn back politics cannot be supported.
"Not only Russia, but also all who cherish the world, especially a world without nuclear weapons, must declare this."
Pulling out of the treaty and threatening to develop nuclear weapons "would be a very dangerous step,” Russia’s Deputy Foreign Minister Sergei Ryabkov was quoted as telling state news agency Tass.
He added a US withdrawal “will cause the most serious condemnation from all members of the international community who are committed to security and stability”.
Meanwhile Konstatin Kosachev, head of the foreign affairs committee in Russia's upper house of parliament, said that a US withdrawal from the treaty would mean "mankind is facing full chaos in the nuclear weapons sphere".
Has Russia violated the terms of the treaty?
In 2017, White House national security officials said Russia had deployed a cruise missile in violation of the treaty.
While the Obama administration accused the Russians of violating the pact by developing and testing a prohibited cruise missile.
Russia has repeatedly denied that it has violated the treaty and has accused the United States of not being in compliance.
What is the UK's stance on this?
Defence Secretary Gavin Williamson has said the UK stands "absolutely resolute" with the US on the treaty dispute.
Mr Williamson also blamed Russia for endangering the arms control pact and he called on the Kremlin to “get its house in order”.
What have experts said?
Mr Trump's decision to withdraw the US from the treaty is likely to restart the arms race between America and Russia, a former UK ambassador to Moscow has said.
Sir Tony Brenton said that despite tensions between the US and Russia "there were hopes" that the two "could find ways to talk about really important issues" such as "strategic nuclear arms".
He continued: "This abandonment of the INF Treaty makes it much, much less likely that they'll be able to have talks on getting the levels of nuclear arsenals down.
"Both countries are in fact talking about producing more nuclear weapons, so we're going in the reverse direction from the one we were going in at the end of the Cold War."
Sir Tony predicted that this could not only lead to an arms race between the US and Russia, but other countries would see nuclear weapons being created and feel the need to protect themselves.
He pointed to North Korea and Iran as two countries which could also possibly begin developing their own nuclear arsenals.
While the US has not said how quickly it plans to withdraw from the agreement, Sir Tony expects this to be "quite rapid... and then the Russians will feel the need to be seen to respond in some way - either by developing weapons or threats of one sort or another, and quite where it goes from there is quite worrying".
Despite his fears that a nuclear arms race will follow the collapse of the treaty, Sir Tony said "there is a lot of justification for what the US has done".
He said that "evidence" that Russia was violating the terms of the pact "came about three or four years ago", and at the time US tried to "deal with it quietly" by going to the Russians and saying "we know you've got this stuff".
However, Sir Tony said the "Russians continued to deny" the allegations "and produced accusations of their own against the US", the validity of which were "dubious".
He continued that this left the "Americans in the position of observing a treaty which the Russians are breaching" which is "very bad if you want to be viewed as a serious foreign policy actor, very bad if you want the Russians to take you seriously and very bad in terms of your reputation worldwide".
However, Sir Tony said that Mr Trump's decision to now pull out of the treaty was a "very dangerous extra twist to the spiral" of increasing tensions between Washington and Moscow.
Despite his predictions of an arms race, Sir Tony said that people would still be able to sleep at night, since nuclear weapons have not been used on civilians since the bombings of Hiroshima and Nagasaki at the end of the Second World War and since then "no one has used a nuclear weapon in anger.
"We have managed to restrain ourselves from using nuclear weapons, I hope that wisdom will continue to prevail."
Are other countries taking the same line as Britain?
Unlike the UK, Germany has called Mr Trump’s move “regrettable”.
The country's Foreign Minister Heiko Maas said in a statement that the three-decades-old INF Treaty is “an important pillar of our European security architecture” and Mr Trump’s announcement “raises difficult questions for us and Europe”.
Mr Maas said Germany has repeatedly urged Moscow to “clear up the serious allegations of breaching the INF treaty, which Russia has so far not done”.
He said Germany is urging Washington to “consider the possible consequences” of its decision, including for a US-Russian nuclear disarmament treaty beyond 2021.
What happens next?
Mr Trump is sending his national security adviser, John Bolton, to Moscow for meetings with Russian leaders, including Foreign Minister Sergei Lavrov and Security Council Secretary Nikolai Patrushev, on Monday and Tuesday.
During his meetings, Mr Bolton is expected to relay the news about the President’s decision.
Mr Bolton is reported to have been urging Mr Trump to withdraw from the treaty before the President made his announcement.
What was the relationship between the US and Russia like before Mr Trump's announcement?
US-Russia relations are already strained as a result of the Ukrainian crisis, the war in Syria and allegations of Russian meddling in the 2016 presidential race and upcoming US midterm elections.
The US has also backed the UK over the Salisbury Novichok nerve agent attack, which British authorities claim Russia is behind.