* Moscow, Prague in biggest row in post-Communist era
* Prague demands its diplomats be allowed to return
* Czech authorities say more Russians could be expelled
* Moscow vows to retaliate if Prague takes action
* Row over 2014 deadly blast at Czech ammunitions depot
* Russian suspects also accused of 2018 poisoning (Adds Kremlin, NATO comments)
By Gabrielle Tétrault-Farber
MOSCOW, April 22 (Reuters) - Russia said on Thursday it would retaliate against any new punitive steps by the Czech Republic against Russian diplomatic staff as a spy row between the two countries threatened to escalate into a new round of expulsions.
Russia ordered out 20 Czech diplomats on Sunday, after Prague expelled 18 Russians the day before. On Wednesday the Czech Republic demanded that Moscow allow the return of all 20 staff to Moscow by Thursday or face further evictions of its diplomats from Prague.
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.
The Czechs say the loss of the 20 staff has effectively paralysed the functioning of their Moscow embassy, which is much smaller than the Russian mission in Prague.
Maria Zakharova, spokeswoman for the Russian foreign ministry, accused Czech authorities of staging a "performance" and pledged Moscow would hit back with an "appropriate response" if Prague took any more actions against Russian diplomats.
Prague expelled the 18 Russians, whom it identified as intelligence officers, after saying two Russian spies accused of a nerve agent poisoning in Britain in 2018 were behind a fatal explosion at a Czech ammunition depot four years earlier.
Russia has denied the accusations, which the Kremlin described as "provocative and unfriendly".
The two suspects named by Prague, 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. (Reporting by Maxim Rodionov, Tom Balmforth and Dmitry Antonov; additional reporting by Robin Emmott in Brussels; writing by Gabrielle Tétrault-Farber; editing by Mark Trevelyan and Gareth Jones)
Our Standards: The Thomson Reuters Trust Principles.