NATO and Ukraine are calling on Russia to stop its "continued and deliberate destabilization" of eastern Ukraine.
In a joint statement issued after meeting in Turkey on May 13, NATO and Ukraine's foreign ministers condemn Russia's "ongoing and wide-ranging military build-up in Crimea," which was annexed by Moscow in March 2014.
They also express concerns about Russia's efforts to build-up its military presence in the Black Sea region.
"This could have further implications on regional stability," NATO Secretary-General Jens Stoltenberg told a press conference.
More than 6,000 people have been killed since fighting began in April 2014 between government forces and separatists in eastern Ukraine, which continues to be embroiled in sporadic violence despite a cease-fire sealed in February.
Kyiv and the West accuse Russia of arming rebels and sending troops in eastern Ukraine, charges Moscow denies.