Russia vowed to retaliate in kind for the decisions by the U.S. and 14 of its European allies to expel scores of Moscow’s diplomats in the wake of the nerve-agent attack on a former spy in Britain.

“This unfriendly step by this group of countries won’t pass without impact and we will respond,” Russia’s Foreign Ministry said in a website statement, accusing the U.K.’s allies of “blindly following the principle of Euro-Atlantic unity.” Kremlin spokesman Dmitry Peskov called the moves “mistaken” and said Russia’s response “will be guided by the principle of reciprocity.” President Vladimir Putin will make the final decision on retaliation, he said.

The U.S. and its allies announced the expulsions Monday, accusing Russia of responsibility for the poisoning of former double agent Sergei Skripal and his daughter outside London with a military-grade nerve agent. The U.S. gave 60 Russian envoys a week to leave and ordered the consulate in Seattle closed, while allies including France, Germany, Poland and Italy expelled dozens of Russian diplomats as well. The moves were the most sweeping coordinated diplomatic action by the U.S. and its allies against Russian in years. Russia denies any role in the Skripal attack.

“Relations are crashing worse than they did in the Cold War,” said Fyodor Lukyanov, head of Russia’s Council on Foreign and Defense Policy, a research group that advises the Kremlin. “This kind of multilateral expulsion is unprecedented.”

Among those given a week to leave the U.S. were 12 diplomats from Russia’s mission to the United Nations in New York.

“This is more serious than it was predicted. Everything now depends on Russia’s will to escalate,” said Vladimir Frolov, a former diplomat and foreign-affairs analyst in Moscow. “The U.S. measures included Russia’s U.N. mission, and this is an escalation,” he said, noting that such large numbers had been expelled last at the height of Cold War tensions in the 1980s. He said that Russia would likely retaliate for the closing of the consulate by ordering the U.S. to leave a diplomatic facility, as well.

“Increasingly, diplomacy is becoming irrelevant in Russian-U.S. relations,” Dmitri Trenin, head of the Carnegie Moscow Center, wrote in Twitter.

Senior members of Russia’s parliament said they expected Moscow to respond with tit-for-tat expulsions of diplomats from each country that ousted its envoys.

The U.S. and Russia have engaged in multiple diplomatic retaliations in the past 15 months. The Obama administration expelled 35 Russian diplomats and shut down two embassy compounds that it said were used for “intelligence-related purposes” in December 2016 in response to alleged Kremlin hacking of the presidential elections won by Donald Trump.

Putin held back from an immediate response, a decision praised as “very smart” by Trump, who’d campaigned on a pledge of improved relations with the Kremlin leader. But after legislators passed a law last July that prevented Trump from easing sanctions without congressional approval, Putin ordered the U.S. to cut staff at its diplomatic missions in Russia by 755, or nearly two-thirds by Sept. 1, to create parity with the number of Russian embassy personnel in the U.S.

The State Department hit back at that decision by directing Russia to close three buildings in the U.S., including its Consulate General in San Francisco, by Sept. 2. Putin warned three days later that he may demand further reductions in American personnel in Russia, saying that 155 of 455 accredited Russian diplomats in the U.S. were assigned to work at the United Nations and were not strictly part of the embassy staff in the U.S.

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