Russia's foreign ministry on Wednesday ordered the expulsion of seven diplomats from Slovakia, Lithuania, Latvia and Estonia, in retaliation for the expulsion of Russian envoys.
The ministry gave the three Slovak and two Lithuanian diplomats, as well as an envoy from Latvia and another from Estonia one week to leave Russia.
Russia accused the four countries of showing "pseudo-solidarity" with the Czech Republic, which ordered most Russian diplomatic staff in Prague to leave last week after accusing Russian spies of being behind a 2014 blast at an ammunition depot. Russia has dismissed the accusations as absurd.
Moscow and Prague are locked in their biggest row since the end of the Communist era in 1989.
The two suspects named by Prague in connection with the 2014 ammunition depot explosion, known under the aliases Ruslan Boshirov and Alexander Petrov, have been reported to be part of the elite Unit 29155 of Russia's GRU military intelligence service.
Britain charged the pair 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. Russia denied involvement in that incident.