Russia has issued a threat of retaliation after Lithuania recently banned sanctioned Russian goods passing through the country to the Russian exclave of Kaliningrad. Dmitry Peskov has described the situation as "more than serious".
Russia has issued a warning to Lithuania - a Nato member - that it will "take action" unless the blockade on moving Russian goods by rail to the Kaliningrad exclave is restored.
Kremlin spokesperson Dmitry Peskov said: "The situation is more than serious. This decision is really unprecedented. It's a violation of everything."
Russia's foreign ministry described the actions of Lithuania as "openly hostile".
It said: "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."
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 the Second World War. Kaliningrad is Russia's only ice-free Baltic port and home to the Russian Baltic fleet, it has a population of approximately 430,000.
It sits between Poland and Lithuania - both Nato members - and is isolated from the rest of Russia apart from by sea. Kaliningrad receives goods via Lithuania and Belarus; there is no transit through Poland.
Lithuania has said that it is simply implementing EU sanctions, part of the measures intended to punish Russia and President Vladimir Putin over the invasion of Ukraine.
Foreign minister Gabrielius Landsbergis said: "It's not Lithuania doing anything, it's European sanctions that started working from 17 June. It was done with consultation from the European Commission and under European Commission guidelines."
On Saturday, Lithuania's state-owned railway told clients that sanctioned goods such as iron and steel would no longer be allowed to cross through Lithuania.
Similar sanctions will follow from 10 July, when no alcohol or concrete goods will be allowed through, from 10 August the same will be implemented with coal, and from December no Russian oil will be permitted to pass through EU territory.
The governor of Kaliningrad, Anton Alikhanov, said that he estimated that the ban would affect approximately 50 percent of all goods heading towards Kaliningrad by rail.