The Estonian foreign ministry said on Friday it had expelled two Russian diplomats and a local news portal reported they were posted at Moscow's consulate in the northeastern town of Narva on the Estonian-Russian border.
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"This is one more unfriendly and groundless action that will not go unanswered," TASS agency cited a Russian foreign ministry official as saying in an initial reaction from Moscow.
The Estonian foreign ministry gave no other details. "We can only confirm that two diplomats have been expelled," said spokesperson Mariann Sudakov.
Delfi, the local Estonian news portal, cited unidentified sources for its report on the two diplomats.
Estonia, a former Soviet republic that is now a member of NATO and the European Union, has a tense relationship with its large neighbor Russia.
Earlier in May, an Estonian court handed out a five-year prison sentence to a Russian national for spying for Russia's main intelligence directorate, the foreign intelligence unit of the Russian armed forces.