The European Union's top diplomat says Russia has concentrated more than 150,000 troops on Ukraine's border and in annexed Crimea.
Josep Borrell declined to give a source for the figure, but said that the risk of further escalation is evident.
''We have to commend Ukraine for its restrained response and we urge Russia to de-escalate and to diffuse tensions."
He said no new economic sanctions or expulsions of Russian diplomats were planned for the time being,
after Ukraine's foreign minister called on the bloc to impose new sanctions on Russia on Monnday.
Tensions between Moscow and Kyiv have been on a knife's edge amid the military build-up.