Bulgaria summoned Moscow's ambassador on Friday after two men based at the Russian embassy in Sofia were accused of spying and ordered to leave the country.

A consular first secretary and an official at Russia's trade representation, both men, collected "state secrets in order to transfer it to a foreign state or organisation," the chief prosecutor's office said.

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The first secretary collected information on elections from 2017, while the trade official gathered sensitive information on the energy sector and energy security from October 2018, they said.

Earlier on Friday, Bulgarian Foreign Minister Ekaterina Zaharieva said Sofia was likely to expel the two Russian diplomats.

The Russian embassy in Sofia said in a statement that the two men had been ordered to leave Bulgaria and would do so.

However, it added that "no evidence was presented as proof of [their] activities" and that Russia reserved the right to reciprocal measures.

"We also consider the decision of the Bulgarian authorities to announce this information publicly before officially notifying the embassy as incompatible with the traditionally constructive spirit of our relations," the statement added.

Bulgarian prosecutors had earlier described both men as diplomats but the Russian statement suggested that only the consular first secretary had this status.

In October, Bulgaria expelled another first secretary at the Russian embassy for gathering classified information on Bulgaria, the European Union and NATO.

Bulgaria, a staunch Soviet satellite during communism, largely maintained its close cultural, historical and economic ties with Russia after the fall of the communist regime in 1989.

The NATO and EU member refused to expel Russian diplomats over the poisoning of former double agent Sergei Skripal and his daughter Yulia in the UK in March 2018.

Friday's accusations come just a day after Sofia charged three Russians in absentia with attempted murder over the 2015 poisoning in Sofia of arms manufacturer Emiliyan Gebrev, his son and his company manager.

The Gebrev and Skripal cases have been linked by the investigative Bellingcat website through the presence of a high-ranking Russian military intelligence officer it named as Denis Vyacheslavovich Sergeev.

In February last year, Bellingcat said Sergeev visited Bulgaria at the same time as Gebrev was poisoned, and that he also entered the UK two days before the Skripal poisoning.