Bulgaria on Thursday announced the expulsion of a Russian diplomat after prosecutors said they suspected Russian involvement in four explosions at arms depots in Bulgaria.
At a meeting with the Russian ambassador, Foreign Minister Ekaterina Zaharieva requested full Russian cooperation with Bulgaria's investigation of the explosions and of attempts to poison Bulgarian arms trader Emilian Gebrev.
Moscow, which has dismissed the Bulgarian probe, said it would respond to the expulsion.
The Bulgarian move follows a series of diplomatic disputes between Russia and other former Soviet-bloc countries in the wake of Czech accusations that Russia was behind a blast in an arms depot in 2014.
Bulgaria, once the Soviet Union's closest ally in eastern Europe, has expelled eight other Russian diplomats for alleged spying since October 2019.
Zaharieva said Sofia reserved the right to take further measures and demanded that Russian diplomats refrain from actions incompatible with their status.
"Bulgaria wants to keep equal and mutually beneficial relations with Russia and that is why it insists on an active and result-orientated cooperation from Russia to shed light on the circumstances around the incidents," the Foreign Ministry said in a statement.
On Wednesday, prosecutors said they had reason to believe there was a link between the blasts in Bulgaria that took place between 2011 and 2020, the attempted murder of Bulgarian arms trader Emilian Gebrev and the blast in the Czech Republic.
Sofia has charged three Russians, which it said were very likely to belong to Russia's GRU military intelligence service, with the poisoning of Gebrev and two other Bulgarians in 2015.
Prague has also accused GRU agents of carrying out the Czech blast in 2014.
Moscow has rejected the Czech allegations as absurd.