The Czech Republic on Thursday ordered Russia to remove most of its remaining diplomatic staff from Prague in an escalation of the worst dispute between the two countries in decades. Moscow said it would swiftly respond.

The spy row flared on Saturday when Prague expelled 18 Russian staff, whom it identified as intelligence officers.

It said two Russian spies accused of a nerve agent poisoning in Britain in 2018 were also behind an explosion at a Czech ammunition depot in 2014 that killed two people.

Russia has denied the Czech accusations and on Sunday ordered out 20 Czech staff in retaliation.

Thursday’s decision, announced by Foreign Minister Jakub Kulhanek, requires Russia to match the number of Czech envoys in Moscow, meaning Russia will have to pull around 20 diplomats and dozens of other staff from Prague by the end of May.

“We will put a ceiling on the number of diplomats at the Russian embassy in Prague at the current level of our embassy in Moscow,” Kulhanek said.

“I do not want to needlessly escalate…but the Czech Republic is a self-confident country and will act as such. This is not aimed against Russians or the Russian nation, but a reaction to activities of Russian secret services on our territory.”

Russian Foreign Ministry spokeswoman Maria Zakharova said Moscow would respond shortly.

“At the moment Prague is on the path to destroying relations,” she said at her weekly briefing.

At a time of acute tensions in Russia’s relations with the West, the dispute has prompted NATO and the European Union to throw their support behind the Czech Republic, which is a member of both blocs.

“Allies express deep concern over the destabilising actions Russia continues to carry out across the Euro-Atlantic area, including on alliance territory, and stand in full solidarity with the Czech Republic,” NATO’s 30 allies said in a statement.

Kremlin spokesman Dmitry Peskov said Moscow took a negative view of Prague’s “hysteria”.

In the past week, Moscow has also kicked out diplomats from Bulgaria, Poland and the United States in retaliation for expulsions of its own staff.

President Vladimir Putin warned foreign powers in his state of the nation speech on Wednesday not to cross Russia’s “red lines”, saying Moscow would make them regret it.

CZECHS SAY THEIR EMBASSY PARALYSED

The Czechs say the loss of the 20 staff has effectively paralyzed the functioning of their Moscow embassy, smaller than the Russian mission in Prague.

The Russian embassy is by far the largest foreign mission in Prague, an overhang since the pre-1989 communist era, and about double the U.S. Embassy.

The Czechs have five diplomats and 19 other staff in Moscow, the Foreign Ministry said on Wednesday, while Russia has 27 diplomats and 67 other staff in Prague after the previous expulsions. That would mean the Czechs ordered a reduction of 70.

A ministry spokeswoman said the decision included both diplomats and other staff, but did not spell out the reduction in exact numbers on Thursday.

The Czech counterintelligence service has repeatedly said that the mission served as a base for intelligence work and its size made it difficult to reduce these activities.

The two suspects named by Prague in connection with the 2014 ammunition depot explosion, known under the aliases Ruslan Boshirov and Alexander Petrov, are reportedly part of the elite Unit 29155 of Russia’s GRU military intelligence service.

Britain charged them in absentia with attempted murder after the poisoning of former Russian spy Sergei Skripal and his daughter with the nerve agent Novichok in the English city of Salisbury in 2018.

The Skripals survived, but a member of the public died. The Kremlin denied involvement in the incident.