Moscow has reacted angrily to restrictions imposed by Lithuania on rail transport between the Baltic Sea exclave of Kaliningrad - which belongs to Russia - and the rest of Russian territory.
The "incipient blockade" of Kaliningrad violates international law, wrote Konstantin Kosachev, a leading foreign policy figure and deputy head of Russia's Federation Council, on his Telegram channel overnight.
The exclave of Kaliningrad lies between Lithuania and Poland.
"As an EU member state, Lithuania is violating a whole series of legally binding international legal acts," Kosachev wrote. For example, he said, the partnership agreement between the European Union and Russia stipulates that neither side will interfere with the other's transport networks.
Since Saturday, Lithuania has banned the rail transit of goods on Western sanctions lists through its territory to Kaliningrad. Western countries have imposed wide-ranging sanctions on Russia since the invasion of Ukraine in late February.
Calls to establish 'a corridor'
Kaliningrad Governor Anton Alikhanov said in a video address on Saturday that "these steps are illegal and may entail far-reaching implications for Lithuania and the European Union."
The ban affected 40-50% of all transit goods, such as building materials and metals, he said.
For weeks, calls have been voiced on Russian state television for Russia to establish a "corridor" from main Russian territory to Kaliningrad. This would, in effect, mean a Russian attack on the NATO states of Latvia and Lithuania.