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(CNN) —  

The Russian government is threatening retaliation over one of the largest mass expulsions of its diplomats in history after more than 20 countries, including the United States, backed action by the UK over the poisoning of a former Russian spy in southern England.

According to state media Tass, Foreign Minister Sergey Lavrov claimed the United States pressured and blackmailed other countries to expel Russian diplomats.

Lavrov said Tuesday the expulsions are the “result of colossal pressure and colossal blackmail, which is Washington’s chief instrument in the international scene.”

He added, “Rest assured, we will respond.”

Kremlin spokesman Dmitry Peskov said the government “deeply regretted” the US decision to expel diplomats, and would analyze the situation before submitting a proposal for retaliatory action to President Vladimir Putin.

“We already stated and reconfirm that Russia has never had any relation to this (poisoning). We will be guided by the principle of reciprocity as before,” Peskov said.

The United States announced Monday it would be expelling 60 Russian diplomats and closing the Russian Consulate in Seattle in what is President Donald Trump’s toughest diplomatic move against Moscow since taking office.

NATO also is withdrawing accreditation for seven staff at its Russian Mission and suspending application requests for three more, Secretary General Jens Stoltenberg said.

The maximum size of the Russian mission to NATO would be reduced by 10, bringing the total number down to 20, Stoltenberg said.

When the Obama administration expelled 35 diplomats over Russian meddling in the 2016 election, the Russian government told the United States to cut its diplomatic staff inside Russia by 755.

In response to the latest expulsion of diplomats, Vasily Nebenzia, Russia’s UN representative, accused the United States of “abusing its rights and obligations” as the host of the UN General Assembly, according to Tass.

Nebenzia said the expulsion of diplomats would hamper Moscow’s efforts at the international body.

“Of course this is a blow to our mission, but I think we’ll mobilize,” Nebenzia told Tass.

May: Mass expulsion historic

Canada, Ukraine and member states of the European Union joined the United States on Monday in expelling more than 100 Russian diplomats.

British Prime Minister Theresa May called the worldwide backlash the “the largest collective expulsion of Russian intelligence officers in history.”

“We have no disagreement with the Russian people who have achieved so much through their country’s great history. But President Putin’s regime is carrying out acts of aggression against our shared values,” May said.

The United Kingdom and Russia have been locked in a diplomatic fight since May’s government accused Moscow of organizing the March 4 poisoning of former Russian double agent Sergei Skripal in Salisbury, England.

Skripal and his daughter, Yulia, were both exposed to a potentially deadly nerve agent, according to British authorities, and remain hospitalized in critical condition.

“We assess that more than 130 people in Salisbury could have been potentially exposed to this nerve agent,” May said Monday, adding that the Skripals may never fully recover.

Russia has denied it was involved in the Salisbury attack and has even suggested the UK could have been behind the poisoning.

US protests Russia’s ‘destabilizing actions’

As of Tuesday morning, 13 Russian diplomats were expelled from Ukraine, while Canada, Germany, France and Poland each expelled four.

More than a dozen other countries, including Australia, Italy and the Netherlands, have each expelled between one and three Russian diplomats or intelligence officers.

European Council President Donald Trusk wouldn’t rule out the possibility of further expulsions of diplomats in the days to come.

New Zealand said it would also expel Russian spies – if it had any.

“While other countries have announced they are expelling undeclared Russian intelligence agents, officials have advised there are no individuals here in New Zealand who fit this profile,” Prime Minister Jacinda Ardern said in a statement Tuesday. “If there were, we would have already taken action.”

The European Council has joined the United Kingdom and the United States in saying the poisoning of Skripal and his daughter was almost certainly the work of the Russian government.

Senior Trump administration officials told CNN the decision to expel the diplomats wasn’t just taken in response to the attack but also in protest to Russia’s “steady drumbeat of destabilizing actions.”

The mass expulsion comes less than a week after Trump called Putin to congratulate him on his re-election, reportedly against the recommendations of his advisers.

Other nations decline to expel Russians

A handful of EU countries suggested they wouldn’t be following suit and expelling Russian diplomats, among them Austria and Portugal.

Austrian Chancellor Sebastian Kurz tweeted that while he stood behind the European Council’s decision to recall the EU ambassador from Moscow, Austria wanted to be “bridge-builders between East and West and keep communication channels to Russia open.”

Similarly, Portugal expressed its “solidarity with the United Kingdom” over the Salisbury attack, though stopped short of expelling Russian diplomats in a statement.

Turkey also said it would not be taking action, with Deputy Prime Minister Bekir Bozdag telling reporters in Ankara: “There is a positive and good relation between Turkey and Russia.”

China, a key ally of Russia’s, has urged countries to “abandon a Cold War mentality.” Foreign Ministry spokeswoman Hua Chunying told reporters Tuesday that “relevant countries” should “avoid taking any actions that would aggravate the conflict.”

CNN’s Sheena McKenzie, Allie Malloy and Angela Dewan contributed to this article.