Britain stands "absolutely resolute" with the United States, Gavin Williamson has said, as US President Donald Trump announced he would pull out of a decades-old nuclear weapons pact with Russia.
The Defence Secretary blamed Russia for endangering the Intermediate-Range Nuclear Forces (INF) Treaty, which was agreed by the US and Soviet Union in 1987, and called on the Kremlin to "get its house in order".
The INF was signed by Ronald Reagan and Mikhail Gorbachev and prohibits the nations from possessing, producing or test-firing ground-launched cruise missiles with a range between 300 and 3,400 miles.
Mr Trump accused Moscow of violating the treaty "for many years" and warned that unless Russia and China, which is not a signatory of the INF, cease developing or possessing the weapons then the US will restart its own programme.#
Russia has been accused of developing new missiles in contravention of the INF several times in recent years.
Mr Williamson accused Russia of breaking the agreement, saying Moscow had made a "mockery" of the INF.
"Our close and long-term ally of course is the United States and we will be absolutely resolute with the United States in hammering home a clear message that Russia needs to respect the treaty obligation that it signed," he told the Financial Times.
The Defence Secretary is in the US while the Royal Navy aircraft carrier HMS Elizabeth visits New York.
"We of course want to see this treaty continue to stand but it does require two parties to be committed to it and at the moment you have one party that is ignoring it," Mr Williamson said.
"It is Russia that is in breach and it is Russia that needs to get its house in order."
Mr Trump did not detail any recent violations of the pact, although in 2017 White House national security officials said Russia had deployed a cruise missile in violation of the treaty.
The Kremlin was also accused of breaking the agreement by former president Barack Obama's administration.
"We're not going to let them violate a nuclear agreement and go out and do weapons and we're not allowed to," Mr Trump said after a rally in Nevada.
"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."
Mr Trump's national security adviser, John Bolton, is due to meet with Russian foreign minister Sergei Lavrov and Kremlin security council Secretary Nikolai Patrushev on a visit to Moscow over the weekend.
The meeting is billed as a continuation of discussions that began when the leaders met in Helsinki in July.
Additional reporting by Press Association.