UPDATE 3-Russia calls US an adversary, warns its warships to avoid Crimea

* Moscow calls US 'an adversary' not a partner

* Warns U.S. warships to stay away from Crimea

* Moves 15 ships to Black Sea for drills

* Shrugs off U.S. warnings of consequences for actions (Adds Ryabkov quotes, details)

By Andrew Osborn and Alexander Marrow

MOSCOW, April 13 (Reuters) - Russia on Tuesday called the United States an adversary and told U.S. warships to stay well away from Crimea "for their own good", calling their deployment in the Black Sea a provocation designed to test Russian nerves.

Moscow annexed Crimea from Ukraine in 2014 and two U.S. warships are due to arrive in the Black Sea this week amid an escalation in fighting in eastern Ukraine, where government forces have battled Russian-backed separatists in a conflict Kyiv says has killed 14,000 people.

"The United States is our adversary and does everything it can to undermine Russia's position on the world stage," Deputy Foreign Minister Sergei Ryabkov was cited as saying by Russian news agencies.

"We do not see any other elements in their approach. Those are our conclusions," the agencies quoted him as saying.

The comment suggests that the veneer of diplomatic niceties that the former Cold War enemies have generally sought to observe in recent decades is wearing thin.

U.S. President Joe Biden said in March that he thought his Russian counterpart Vladimir Putin was a killer who would "pay a price" for alleged meddling in U.S. elections - an accusation that Moscow denies.

Ryabkov's remarks suggest Russia will in turn robustly push back against what it sees as unacceptable U.S. interference in its own backyard.

"We warn the United States that it will be better for them to stay far away from Crimea and our Black Sea coast. It will be for their own good," said Ryabkov.

The West is sounding the alarm over what it says is a large unexplained build-up of Russian forces close to Ukraine's eastern border and in Crimea, which NATO Secretary-General Jens Stoltenberg on Tuesday called on Moscow to unwind.

Russia has said it moves its forces around as it sees fit, including for defensive purposes, and has regularly accused NATO of destabilising Europe by moving its military infrastructure closer to Russia's borders.

'STAY AWAY'

Ryabkov was cited as shrugging off U.S. talk of consequences for any "aggressive" Russian actions and as saying that Moscow had studied U.S. tactics towards Russia and adapted accordingly.

U.S. military support to Kyiv was a serious challenge for Russia, he added, accusing Washington and NATO of turning Ukraine into a "powder keg" with increasing arms supplies.

Washington says Ukraine needs a strong army to defend itself from potential Russian aggression.

"Any threat to us merely confirms our belief that our course is the right one," Ryabkov was quoted as saying, warning U.S. warships in the Black Sea to keep their distance, given what he said was the high risk of unspecified incidents.

"There is absolutely nothing for American ships to be doing near our shores, this is purely a provocative action. Provocative in the direct sense of the word: they are testing our strength, playing on our nerves. They will not succeed," Ryabkov said.

The Pentagon has declined to discuss the ships' deployment, saying only that the U.S. military routinely sends vessels to the region.

Russia's Black Sea Fleet is based in Crimea and it has powerful missile and radar facilities on the peninsula.

Russia confirmed on Tuesday it was continuing to move 15 navy vessels to the Black Sea from the Caspian Sea to take part in drills. (Additional reporting by Maxim Rodionov; Editing by Jon Boyle and John Stonestreet)