The Czech government said on Thursday it would slap a ceiling on the Russian embassy workforce as part of a spiralling diplomatic row, which means dozens of staff will have to leave by next month.
Foreign Minister Jakub Kulhanek said Russia will have until the end of May to withdraw 63 embassy employees to reduce their numbers to the same level as those of the Czech embassy in Moscow.
Prague currently has a total of 32 embassy staff in Russia, which in turn has 95 staff in Prague, according to updated figures from the Czech foreign ministry.
Russia threatened retaliation, but Kulhanek later said Moscow had agreed to follow the principle of diplomatic parity.
"In line with article 11 of the Vienna Convention (on Diplomatic Relations), we will limit the number of Russian embassy staff in Prague to the actual number at our embassy in Moscow," Kulhanek told reporters.
Prague expelled 18 diplomats earlier this week over a Czech intelligence statement that Russian spies orchestrated a fatal explosion on Czech soil in 2014.
Moscow kicked out 20 embassy staff in retaliation.
Kulhanek gave Moscow an ultimatum Wednesday to allow the 20 Czechs to return to work by Thursday noon (1000 GMT), but Russia ignored the call.
"Russia's reaction was absolutely disproportionate, the diplomats didn't do anything wrong," Kulhanek said.
The 1961 Vienna Convention says that if there is no agreement on the size of embassies between two countries, the receiving state may keep the foreign mission "within limits considered by it to be reasonable and normal".
- 'Deep concern' -
The Russian foreign ministry, which summoned Czech ambassador to Moscow Vitezslav Pivonka for Thursday afternoon, reacted angrily.
"Prague decided to take the road to destroy the relation and won't have to wait long for our answer," Russian foreign ministry spokeswoman Maria Zakharova told reporters.
But Kulhanek told Czech TV later that Russia "announced it was switching to strict parity as regards the workforce at the Russian and Czech embassies. The numbers should be the same".
Ambassador Pivonka said after being summoned that he was staying in Moscow and that he expected further talks on bilateral relations in the future.
The EU and NATO voiced support to Prague, while neighbouring Slovakia, which formed a single country with the Czech Republic until 1993, said it would expel three Russian diplomats.
It said the move was due to both solidarity with Prague and information received from its intelligence services.
- 'Beyond established rules' -
"In the case of these three persons, their activities went beyond established rules in an international environment," Slovak Defence Minister Jaroslav Nad told reporters on Thursday.
Last Saturday, Prague accused the Russian foreign intelligence service of being behind an explosion at an ammunition depot near the eastern village of Vrbetice in 2014 that killed two Czech nationals.
Police are seeking two men in connection with that blast, as well as a second, non-fatal explosion nearby later that year.
Britain has identified the same men as suspects in the 2018 poisoning of former Russian double agent Sergei Skripal in Salisbury, England.
Some media reports have suggested the arms may have been destined for Ukraine.
The explosions occurred the same year that Russia annexed the Crimean peninsula from Ukraine and a conflict broke out between Ukrainian forces and Russia-backed rebels in the east of the country.