Russia on Sunday said it would retaliate against the Czech government's "unprecedented" decision to expel 18 Russian diplomats identified as secret agents and summoned the Czech ambassador.
"We will take retaliatory measures that will force the authors of this provocation to understand their full responsibility for destroying the basis of normal relations between our countries," the Russian foreign ministry said in a statement.
The ministry did not specify which measures would be taken, but called the Czech announcement "absurd".
A source told AFP on Sunday that the Czech ambassador in Russia had been summoned to the foreign ministry.
The Russian foreign ministry did not release any details.
On Saturday, Czech authorities said they would expel 18 Russian diplomats identified by local intelligence as secret agents of the Russian SVR and GRU security services suspected of involvement in a 2014 explosion.
Czech police also said they were seeking two Russians in connection with the blast that killed two people, and who carried passports used by suspects in the attempted poisoning of former Russian double agent Sergei Skripal in 2018.
Moscow called the announcement a "hostile step" and alleged it bore a "trace" of involvement by Washington.
"In their desire to please the United States against the background of recent US sanctions against Russia, Czech authorities in this respect even outdid their masters from across the pond," the ministry added.
Earlier this week the United States announced sanctions and the expulsion of 10 Russian diplomats in retaliation for what Washington said was the Kremlin's US election interference, a massive cyber attack and other hostile activity.