LONDON (REUTERS) - Russia warned Nato member Lithuania on Monday (June 20) that unless the transit of goods to Russia's Kaliningrad exclave on the Baltic Sea was swiftly restored then Moscow would take undisclosed measures to defend its national interests.

With east-west relations at a half-century low over Russia's Feb 24 invasion of Ukraine, Vilnius banned the transit of goods sanctioned by the European Union through Lithuanian territory to and from the Russian exclave, citing EU sanction rules.

Russia's foreign ministry summoned Lithuania's top envoy in Moscow to deliver a protest while the Kremlin said the situation was beyond serious.

"The situation is more than serious," Kremlin spokesperson Dmitry Peskov told reporters. "This decision is really unprecedented. It's a violation of everything."

Russia's foreign ministry demanded Lithuania reverse what it cast as an "openly hostile" move immediately.

"If cargo transit between the Kaliningrad region and the rest of the Russian Federation via Lithuania is not fully restored in the near future, then Russia reserves the right to take actions to protect its national interests," it said.

Kaliningrad, formerly the port of Koenigsberg, capital of East Prussia, was captured from Nazi Germany by the Red Army in April 1945 and ceded to the Soviet Union after World War Two.

It is sandwiched between the members of the North Atlantic Treaty Organisation (Nato), Poland and Lithuania.

Lithuania said it was merely implementing EU sanctions, part of a swathe of measures intended to punish President Vladimir Putin for the invasion of Ukraine.

"It's not Lithuania doing anything: it's European sanctions that started working from 17 of June," Lithuanian Foreign Minister Gabrielius Landsbergis told reporters in Luxembourg. "It was done with consultation from European Commission and under European Commission guidelines," Landsbergis said.

Lithuania's state-owned railway informed clients that from June 17 sanctioned goods such as steel and iron would not be permitted to cross Lithuania, Landsbergis said.

European Commission Vice President Valdis Dombrovskis said he had spoken to Lithuanian President Gitanas Nauseda who said that Vilnius was applying EU sanctions.

"Clearly we really need to gather all facts and implications, but as President Nauseda outlined, what Lithuania is doing they are applying EU sanctions," Dombrovskis said. "So in this case, indeed, if it is application of the EU sanctions, it's clear that we need to be with our member states applying the sanctions."