The United States on Thursday called on Ukraine and Russia to show restraint after Moscow accused Kiev of plotting attacks in Crimea, and urged a resumption of talks.

The United States on Thursday called on Ukraine and Russia to show restraint after Moscow accused Kiev of plotting attacks in Crimea, and urged a resumption of talks.

Ukraine has put its forces on high alert after Russia's FSB security service said it had thwarted "terrorist attacks" in the Black Sea peninsula this week by Ukrainian military intelligence and beaten back armed assaults -- claims Kiev fiercely denies.

"We are extremely concerned about the increased tension near the administrative boundary between Crimea and Ukraine," said a State Department spokeswoman, Elizabeth Trudeau.

"Our position, as I said yesterday, is well known: Crimea is part of Ukraine and is recognized as such by the international community," she said.

"We call for the avoidance of any actions that would escalate the situation," Trudeau added. "Take a step back."

"We continue to remain in close touch with international partners on this. But we believe now it's the time to reduce the tensions, to reduce the rhetoric and get back to talks."

Moscow and Kiev have been locked in a bitter dispute since the Kremlin annxed Crimea in March 2014 following  a national referendum after Ukraine's Russian-backed president Viktor Yanukovych was ousted.

The latest war of words represents the most serious increase in tensions in months as a separatist conflict in eastern Ukraine drags on despite a stalled peace deal.