Russia has said it would be forced to respond in kind to restore military balance with the US if President Donald Trump followed through on his plan to quit a landmark nuclear arms treaty and began developing new missiles.
Mr Trump drew a warning of "military-technical" retaliation from Moscow after saying on Saturday that Washington would withdraw from the Cold War-era Intermediate-Range Nuclear Forces Treaty, which rid Europe of land-based nuclear missiles.
The INF treaty required the elimination of all short-range and intermediate-range land-based nuclear and conventional missiles held by both countries in Europe.
It was signed by former US President Ronald Reagan and reformist Soviet leader Mikhail Gorbachev in 1987 at a time of unprecedented East-West détente.
Its demise could raise the prospect of a renewed arms race, and Mr Gorbachev has warned that unravelling it could have catastrophic consequences.
Kremlin spokesman Dmitry Peskov has called Mr Trump's withdrawal plan a matter of deep concern for Moscow.
"Such measures can make the world more dangerous," he said during a daily conference call with reporters.
Despite repeated Russian denials, US authorities believe Moscow is developing and has deployed a ground-launched system in breach of the INF treaty that could allow it to launch a nuclear strike on Europe at short notice.
Mr Trump said the United States would develop equivalent weapons unless Russia and China agreed to a halt in development. China is not a party to the treaty.
Mr Peskov said Russian President Vladimir Putin had repeatedly warned that the demise of the treaty would force Moscow to take specific military steps to protect its own security.
"Scrapping the provisions of the INF treaty forces Russia to take measures for its own security because what does scrapping the INF treaty mean?," said Mr Peskov.
"It means that the United States is not disguising, but is openly starting to develop these systems in the future, and if these systems are being developed, then actions are necessary from other countries, in this case Russia, to restore balance in this sphere."
Mr Trump's national security adviser John Bolton is due to hold talks with senior officials in Moscow and to meet Mr Putin on Tuesday.
Mr Peskov said Mr Trump's decision to quit the pact would be a subject for discussion and that Moscow was looking for a detailed explanation as to why Washington had decided to turn its back on the treaty.
The INF treaty required the US and the Soviet Union to forego all nuclear ground-launched ballistic and cruise missiles with a range of 500km to 5,500 km, eliminating an entire category of weapon.
The European Union called the INF treaty "a pillar of European security architecture" which had resulted in the destruction of almost 3,000 nuclear and conventional warheads and continued to play an important non-proliferation role.
Mr Trump's statement has alarmed some European countries.
French President Emmanuel Macron spoke to Mr Trump on Sunday to stress the importance of the treaty, his office said.
The German government said it regretted Mr Trump's decision, saying NATO would now have to discuss the development.
China also condemned Trump's move on Monday, saying it waswrong to unilaterally pull out of the treaty.
In Moscow, Mr Peskov said there was a six-month period for Washington to withdraw from the INF treaty once it had given official notification it was leaving, something he noted it had not yet done.
That meant the question of Russia acting to restore the military balance between Washington and Moscow was not "for today or tomorrow," he said.
Mr Peskov denied US accusations Russia had breached the treaty, alleging that the US was the one at fault and had been steadily undermining it.