Ukrainian troops along the de facto border with Crimea and facing rebels in the east have been put on high alert.
It follows accusations by Russia on Wednesday that Ukraine was carrying out armed incursions into Crimea.
Ukraine’s envoy to the UN asked Russia to provide evidence of the claims, and said some 40,000 Russian troops were massed on the Crimea-Ukraine border.
His Russian counterpart told the UN Security Council of Moscow’s “concern and outrage” at the alleged incursions.
Crimea was annexed by Russia in 2014 after an unrecognized referendum.