Ukraine: Russian forces deploy on the border 0:32
(CNN) --
Russia's military operation in the Donbas region of eastern Ukraine has heightened the importance of US and NATO defenses in Eastern Europe, which for decades have acted as a buffer between Russia and the West. .
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NATO has four battalion-sized multinational battle groups in Estonia, Latvia, Lithuania and Poland, operating on a rotating basis.
Each is run by the UK, Canada, Germany or the US, and contains troops from various member countries.
The alliance said last month that the four groups are "robust and combat-ready forces."
Its secretary general, Jens Stoltenberg, has also said he is prepared to quickly bolster that presence by sending additional forces and supplies to the area.
The Ukraine-Russia conflict explained in 4 maps
And NATO deployments are supported by US troops.
The United States has its own bases in Lithuania and Romania, along with several in Poland.
President Joe Biden formally approved last week the deployment of 3,000 more US troops to Poland, Germany and Romania, with another 8,500 troops on high alert.
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NATO has no troops in Ukraine and no plans have been announced to send alliance troops to the country.
But while Ukraine is not a member of NATO, the alliance also provides strategic-level advice to the country and has described the relationship as "one of the most important partnerships in NATO."
Other assets at NATO's disposal in the region include a missile defense system in Romania, designed to "detect, track, engage and destroy" ballistic missiles in the atmosphere.
A similar facility in Poland is scheduled to come online this year.
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