London/Washington - The United States said on Monday it would expel 60 Russian diplomats, joining governments across Europe in taking action against the Kremlin for a nerve agent attack on a former Russian spy in Britain that they have blamed on Moscow.
It was the toughest action that U.S. President Donald Trump had taken against Russia since coming to office.
British Foreign Secretary Boris Johnson tweeted that "Today's extraordinary international response by our allies stands in history as the largest collective expulsion of Russian intelligence officers ever and will help defend our shared security."
British Prime Minister Theresa May said the coordinated measures "clearly demonstrate that we all stand shoulder to shoulder in sending the strongest signal to Russia that it cannot continue to flout international law".
Russia's Foreign Ministry called the actions a "provocative gesture" and promised to respond.
Moscow has denied being behind the attack on Sergei Skripal and his daughter in the southern English city of Salisbury on March 4.
The staff expelled by the United States included 12 intelligence officers from Russia's mission to United Nations headquarters in New York. Trump also ordered the closure of the Russian consulate in Seattle.
"To the Russian government we say: when you attack our friends, you will face serious consequences," a senior U.S. administration official told reporters, speaking on condition of anonymity.
EU leaders last week said evidence of Russian involvement in the attack presented by British Prime Minister Theresa May was a solid basis for further action.
Germany and France made good on those threats by announcing expulsions and in a coordinated move, other states across the EU followed suit, along with Canada and Ukraine.
Skripal's poisoning, alleged to have employed the Soviet-era military-grade nerve agent Novichok, is the first known offensive use of a nerve toxin in Europe since World War Two.
"In solidarity with our British partners, we have today notified the Russian authorities of our decision to expel four Russian personnel with diplomatic status from French territory within one week," said French Foreign Minister Jean-Yves le Drian.
"RESPONSE WILL BE SYMMETRICAL"
European Council President Donald Tusk said 14 EU countries in all were expelling Russian diplomats and warned that further measures could be taken in the coming weeks and months.
Russia said it would respond in kind.
"The response will be symmetrical. We will work on it in the coming days and will respond to every country in turn," the RIA news agency cited an unnamed Foreign Ministry source as saying.
The Kremlin has accused Britain of whipping up an anti-Russia campaign and sought to cast doubt on the British analysis that Moscow was responsible. The two countries have already expelled 23 diplomats each in tit-for-tat moves.
Russian Foreign Ministry spokeswoman Maria Zakharova suggested that the EU's expression of support for Britain was misguided given that it would be leaving the bloc next year.
"Britain is leaving the European family. No one cancelled Brexit, and the divorce process is in full swing," Russian Foreign Ministry spokeswoman Maria Zakharova wrote on Facebook.
"Therefore a country which is leaving the European Union is exploiting the solidarity factor and is foisting on those countries that remain a worsening of relations with Russia."
Skripal, 66, and his 33-year-old daughter Yulia were found unconscious on a public bench in a shopping centre, and remain in hospital in critical condition.
A British court has said they may have suffered brain damage, while a policeman who went to help them has also indicated that he has suffered lasting damage to his health.
British Defence Secretary Gavin Williamson hailed the support Britain had received during a visit on Monday to Estonia, where Britain has troops as part of a NATO mission to deter any Russian aggression following its seizure of Ukraine's Crimea in 2014.
"The world's patience is rather wearing thin with President Putin and his actions," Williamson said.
"Their aim is to divide, and what we are seeing is the world uniting behind the British stance - and that in itself is a great victory and sends an exceptionally powerful message to the Kremlin and President Putin."