The United States formally withdrew from a key Cold War-era nuclear treaty with Russia.
Many fear that the treaty's collapse will spark a new arms race in Europe and Asia.
UN Secretary-General Antonio Guterres told reporters on Thursday "the world will lose an invaluable brake on nuclear war" and that it "will likely heighten, not reduce, the threat by ballistic missiles".
German Foreign Minister Heiko Maas said on Thursday that "Europe is losing part of its security" but sought to alleviate fears on Friday, telling German radio that the end of the treaty did not imply a renewed global armament race and that Europe would not take part in such a process.
He also called on China and India to be included in a new international treaty.
US President Donald Trump said in February the country would pull out of the Intermediate-Range Nuclear Forces Treaty (INF) on August 2 unless Moscow came back into compliance.
The US and NATO allies placed the blame on Moscow, stating that they failed to destroy a noncompliant missile system.
"Russia is solely responsible for the treaty’s demise," the US secretary of state Mike Pompeo said in a statement. "Dating back to at least the mid-2000s, Russia developed, produced, flight-tested, and has now fielded multiple battalions of its noncompliant missile".
NATO allies echoed the US, stating "Russia today remains in violation of the INF Treaty, despite years of US and Allied engagement, including a final opportunity over six months to honour its treaty obligations. As a result, the United States decision to withdraw from the treaty, a decision fully supported by NATO allies, is now taking effect."
Moscow, on the other hand, blamed the US for the treaty's collapse.
Russia's foreign ministry is quoted by the Sputnik news outlet as saying: "On 2 August 2019 the Treaty between the Union of Soviet Socialist Republics and the United States of America on the elimination of medium- and short-range missiles, signed in Washington on 8 December 1987, was terminated at the initiative of the American side."
A key Cold War-era missile treaty
Moscow and Washington signed the Intermediate-Range Nuclear Forces (INF) Treaty in 1987 but have in recent years accused each other of violating its terms.
The agreement prohibits both countries from owning missiles with ranges between 500 and 5,500 kilometres.
The US said it had evidence that a new Russian missile — known in Moscow as 9M729 but as SSC-8 by NATO allies — has a range of over 500 kilometres, which Russia denies.
"They are dual-capable, they can carry nuclear weapons, they can reach European cities within minutes, they are mobile, hard to detect and they also reduce the threshold of any potential use of nuclear weapons in armed conflict," NATO chief Jens Stoltenberg told reporters on Wednesday, talking about the Russian missiles.
'We will not mirror Russia'
Stoltenberg warned NATO countries "will respond", adding that it will be done in "a defensive, coordinated and measured way".
"We will not mirror what Russia is doing. We don’t intend to deploy new nuclear land-based systems in Europe. But, of course, we have to make sure that also after the demise of the INF Treaty with more Russian missiles in Europe, we need to make sure that we continue to have credible deterrence and defence and we will do what is necessary to maintain credible deterrence and defence," he added.