Lithuania on Thursday accused Russia of a military "invasion" of conflict-torn Ukraine and called for a United Nations Security Council meeting over the issue.

"Lithuania strongly condemns the obvious invasion of the territory of Ukraine by the armed forces of the Russian Federation," the foreign ministry said in a statement.

"Lithuania urges the UN Security Council to discuss this matter immediately."

The Baltic nation, which is a temporary member of the council, called on Russia "to immediately withdraw its military forces, weaponry and equipment from the sovereign territory of Ukraine".

Moscow has repeatedly denied Western allegations that it is sending troops into eastern Ukraine, where government forces have been battling pro-Russian separatists in a now four-month-long conflict.

But Lithuanian Foreign Minister Linas Linkevicius told AFP on Thursday that "it is aggression and undeclared war, despite denials".

"Facts and NATO intelligence information make the denials less and less convincing," he added, calling on the West to provide military support for Ukraine.

The Kremlin was forced to admit Tuesday that a number of its troops captured by Ukrainian forces had crossed the border "by accident".

Ukraine reported Wednesday that a huge convoy of tanks and weaponry from Russia was moving through the southeast of the country, while Poland said its intelligence service and NATO have evidence that regular units of the Russian army are operating in Ukraine.

The three Baltic nations, which spent five decades under Soviet occupation until 1991, have been following the Ukraine events closely, concerned about the impact of Moscow's actions there on their security.

Latvian Foreign Minister Edgars Rinkevics for his part took to Twitter on Thursday to condemn Moscow's actions.

"Russian invasion in Ukraine must be considered by the UN Security Council as act of aggression, UN must react accordingly, this is war," he said.

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