Vladimir Putin may plan to meddle in the Baltic states to "restore Russia to its former position as a great power," former NATO chief Anders Fogh Rasmussen warned, according to the Telegraph.

"There is a high probability that (Putin) will intervene in the Baltics to test NATO's Article 5," said Rasmussen, who headed the Atlantic alliance from 2009 to 2014.

The former secretary-general was referring to a clause of the 1949 North Atlantic Treaty that defines "an armed attack against one or more" of the signees as "an attack against them all." If a NATO country faces aggression, the remaining members of the alliance are required to rush to its defense.

All three of the Baltic states -- Estonia, Latvia and Lithuania -- have been part of the alliance since 2004. As part of the Soviet Union, they were members of the Warsaw Pact from 1955 to 1991.

Rasmussen suggested the Russian president may not be bold enough to directly attack one of the nations. Instead, he pointed to concerns the Kremlin might generate a murky conflict in Estonia or Latvia, countries with large Russian minorities.

"Putin knows that if he crosses the red line and attacks a NATO ally, he will be defeated," he said. "But he is a specialist in hybrid warfare."

In Brussels, U.S. Vice President Joe Biden echoed the comments by accusing Moscow of using "little green men without patches" to help separatists destabilize Ukraine, where fighting between government forces and pro-Russian militants has intensified in recent weeks.

Biden was meeting with European Union officials attempting to stem Russia's incursion into Ukraine, Fox News noted, as French President François Hollande and German Chancellor Angela Merkel continued their high-stakes, top-level diplomatic mission to Kiev and Moscow.

"Ukraine is fighting for their very survival right now," Biden said. "Russia continues to escalate the conflict by sending mercenaries and tanks." Among the troops loyal to Moscow were "very sophisticated special operations soldiers."

Putin's spokesman, Dmitry Peskov, said the conflict in Ukraine was a "civil war," which the Russian president, Hollande and Merkel were all hoping to help end, the Guardian reported.

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