Vice President Biden is questioning Moscow's committment to peace, accusing Russian President Vladimir Putin of sending unmarked soldiers known as "little green men" into Ukraine.

"Putin continues to call for new peace plans as his troops roll through the Ukrainian countryside and he absolutely ignores every agreement that his country has signed in the past and he has signed recently," Biden said Friday at European Union headquarters in Brussels.

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"Ukraine is fighting for their very survival right now," Biden said. "Russia continues to escalate the conflict by sending mercenaries and tanks and, as we euphemistically say in the United States, little green men without patches, who are very sophisticated, special-operations soldiers."

German Chancellor Angela Merkel and French President Fran├žois Hollande are in Moscow on Friday meeting with Putin and pressing for a cease-fire between the Ukrainian government and pro-Russian separatists. Secretary of State John KerryJohn Forbes KerryUN chief warns unchecked climate change will mean 'survival of the richest' Conservatives rip FBI over IG report: 'scathing indictment' Live coverage: DOJ inspector general testifies on Capitol Hill MORE visited Kiev on Thursday.

"Everyone is aware that the first step must be the cease-fire, but that it cannot suffice. We must seek a global solution," Hollande said in Paris before heading to Moscow, according to The Associated Press.

Russia has denied backing the separatists, but as recently as Thursday a top NATO commander said Russia continues to supply them with high-grade weapons, fighters and air defenses.

Biden argued that the U.S. and 28-member EU should stand in solidarity against barriers to trade and other "global challenges."

Biden's remarks came as the White House released Friday an updated version of its National Security Strategy, a 29-page document that highlighted, among other threats, "Russia's violation of Ukraine's sovereignty and territorial integrity" and its "belligerent stance toward other neighboring countries."