Vilnius said Wednesday it was summoning Moscow's diplomatic representative over Russia's plans to unilaterally extend its maritime border into Lithuanian and Finnish waters, warning the move could be part of a hybrid warfare campaign.
Lithuania's foreign ministry said it was "summoning a representative of the Russian Federation for a full explanation".
Lithuania expelled Russia's ambassador and downgraded its diplomatic relations with Moscow in April 2022 in response to atrocities discovered in the Ukrainian town of Bucha.
Finland's Foreign Minister Elina Valtonen meanwhile told reporters that Helsinki was "following the situation".
"We don't have any official information of what Russia is planning," she said.
According to a draft resolution drawn up by the Russian defence ministry and published on Tuesday, Moscow plans to extend its territorial waters by changing its maritime border in the Baltic Sea with Finland and Lithuania.
The redefinition of geographical coordinates would see Moscow declaring Finnish and Lithuanian sea areas as Russian.
Russia's borders in the westernmost Kaliningrad region and the eastern Gulf of Finland would be altered, according to the document.
"The Russian Federation is also a member and party to the UN Convention on maritime borders. We only expect Russia to respect that convention," Valtonen said.
"It should be remembered that causing confusion is also hybrid influence. Finland will not be confused," she wrote earlier on X.
Lithuania's Foreign Minister Gabrielius Landsbergis echoed that idea.
"Another Russian hybrid operation is underway, this time attempting to spread fear, uncertainty and doubt about their intentions in the Baltic Sea," he wrote on X.
"This is an obvious escalation against NATO and the EU, and must be met with an appropriately firm response," he said.
Kremlin spokesman Dmitri Peskov told reporters in a briefing on Wednesday that there was "nothing political" about the move, "although the political situation has changed a lot since 1985" - the year the Russian territorial waters were demarcated.
"You can see what the level of confrontation is at the moment, especially in the Baltic region. This requires the appropriate bodies to take the appropriate measures to ensure our security."
A statement from the Lithuanian foreign ministry "urges (the) Russian Federation to respect and abide by the universally recognised principles and norms of international law, in particular UN Convention on the Law on Sea".
Finnish President Alexander Stubb, who oversees Finland's foreign policy, wrote on X that "Russia had not been in contact with Finland on the matter".
The Russian defence ministry said the move, which was planned to enter into force in January 2025, was necessary because the current coordinates "do not fully correspond to the current geographical situation".