Lithuanian President Gitanas Nauseda on Friday pardoned two Russians jailed by Vilnius for espionage, prompting Moscow to announce it would respond in kind in what media said could be spy swap.
Nauseda signed the decree to pardon Nikolai Filipchenko and Sergei Moisejenko who are serving prison terms for espionage in the Baltic EU and NATO state, according to the statement published on his official website.
Russia's SVR foreign intelligence agency director Sergei Naryshkin told news agencies later on Friday that Moscow would take "reciprocal measures", without going into detail.
Both Russians were sentenced by Lithuanian courts in 2017. They could be exchanged for two Lithuanian citizens, Yevgeny Mataitis and Aristidas Tamosaitis, convicted of spying in Russia, the Baltic News Service said.
A Norwegian, Frode Berg, convicted of spying and jailed in Russia could also be part of the swap, BNS said, quoting senior officials who requested anonymity.
While there was no official comment from Oslo, Berg's lawyer said on Thursday that the swap could come soon.
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"This may happen in one or two days, depending on how quickly practical questions are resolved," Brynjulf Risnes told the Norwegian NTB agency.
Nauseda's decree said the Russians were pardoned in line with a new law on spy swaps.
Presidential spokesman Antanas Bubnelis declined further comment when contacted by AFP on Friday.
Lithuanian officials said Filipchenko worked for the FSB Russian federal security service and was trying to recruit senior officials in the Baltic state, which was under Moscow's thumb during the Soviet era.
He was sentenced to 10 years behind bars and did not appeal. Moisejenko was jailed for 10 years and six months after a court ruled he recruited a Lithuanian army captain who served at the country's Siauliai military air base. He had pleaded innocent.
The two Lithuanians were sentenced in Russia for allegedly sharing Russian military intelligence with Lithuania.
Lithuania and its fellow Baltic states Estonia and Latvia have strained relations with neighbouring Russia.
Moscow occupied and annexed the three countries during World War II. They only broke free from the crumbling Soviet Union in 1991 and joined the European Union and NATO in 2004.
Tensions between the Baltic states and Moscow have been heightened since the outbreak of the Ukraine crisis in 2014.
The trio have since seen a string of espionage allegations involving Russia's intelligence service.
Estonia and Russia swapped convicted spies last year, while in 2015, Russia freed Estonian officer Eston Kohver in a Cold War-style bridge swap between the two countries.
Kohver, who was sentenced by Russia to 15 years in jail on espionage and other charges, was exchanged for Aleksei Dressen, a former Estonian security official serving a 16-year sentence for spying for Moscow, the FSB had said at the time.