Situation at Ukraine's border with the Crimean Peninsula has been calm so far as Moscow tightened security. Kiev put its forces around Crimea on high alert on Thursday. Tensions have soared after Russia's FSB security service said it had thwarted what it called as "terrorist attacks" in Crimea this week by Ukrainian military intelligence.

The FSB also said it had dismantled a Ukrainian spy network inside the peninsula. But Kiev fiercely denied the claims.

A NATO official said the alliance was monitoring the heightened Crimea tensions with concern. The US also called on Russia and Ukraine to show restraint, saying Washington was "extremely concerned."