Russia has warned Donald Trump that the world will become "more dangerous" if he pulls the US out of a Cold War-era nuclear deal.
The US president has announced his intention to quit the Intermediate-Range Nuclear Forces Treaty, which was drawn up to protect America and its allies in Europe and the Far East, because of alleged violations by Moscow.
The agreement – signed by Mikhail Gorbachev and Ronald Reagan in 1987 – is meant to prohibit the countries from possessing, producing or testing any ground-launched cruise missiles with a range of 300 to 3,400 miles.
Former Soviet leader Mr Gorbachev has said it would be a "mistake" if Washington scrapped the treaty.
Now the Kremlin – which denies not sticking to the deal's terms – says America's withdrawal would "make the world a more dangerous place".
"Russia would be forced to take measures to restore the balance of nuclear power in the event of a US exit from the treaty," said a statement from the Russian capital.
"The US intention to possibly exit form the treaty is a cause for deep concern. The US itself is in breach of the treaty."
Moscow also pledged that it would "never attack anyone first" with a nuclear strike, following comments by Vladimir Putin that Russians would "go to heaven" as martyrs in case of such a war.
The Kremlin statement added: "We don't feel that we have the right to inflict the first strike."
Concern over the potential scrapping of the treaty has gone far beyond Russia, with Heiko Maas, German foreign affairs minister, describing the near 30-year-old pact as "an important pillar of our European security architecture".
He said talk of scrapping it "raises difficult questions for us and Europe", with the EU urging the US and Russia to enter a "constructive dialogue" in a bid to preserve the treaty.
French President Emmanuel Macron has also stressed the importance of the deal during a phone call with Mr Trump.
His office said of their conversation on Sunday: "The president noted the importance of this treaty, in particular for European security and our strategic stability."
But UK Defence Secretary Gavin Williamson has said Britain stands "absolutely resolute" with the US over the decision and called on the Kremlin to "get its house in order".
The fallout is expected to be a topic of conversation between US national security adviser John Bolton and senior Russian officials when he visits Moscow this week.
Mr Putin is said to be keen to "seek an explanation" as to why Washington wants to pull out, having denied so-far unsubstantiated claims by Mr Trump that Russia is not adhering to its terms.
Explaining his announcement during a campaign stop in Elko, Nevada, on Saturday, Mr Trump said: "Russia has violated the agreement. They've been violating it for many years
"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."
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American officials previously accused Moscow of violating the treaty by deploying a land-based cruise missile that could allow it to launch a nuclear strike on Europe at short notice.
China is not yet signed up to the pact.
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