Crimea's Moscow-backed leaders declared a 96-percent vote in favor of quitting Ukraine and annexation by Russia in a referendum on Sunday, March 16, that Western powers said was illegal and will bring immediate sanctions, according to Reuters.

As state media in Russia carried a startling reminder of its power to turn the United States to "radioactive ash", President Barack Obama spoke to Vladimir Putin, telling the Russian president that he and his European allies were ready to impose "additional costs" on Moscow for violating Ukraine's territory.

With three-quarters of votes counted in Crimea, a Black Sea peninsula that is home to 2 million people, 95.7 percent had supported annexation by Russia, chief electoral official Mikhail Malyshev, was quoted as saying by local media.

Turnout was 83 percent, he added - a high figure given that many who opposed the move had said they would boycott the vote.

U.S. and European officials say military action is unlikely over Crimea, which Soviet rulers handed to Ukraine 60 years ago. But the risk of a wider Russian incursion, as Putin probes Western weakness and tries to restore Moscow's influence over its old Soviet empire, leaves NATO calculating how to help Kiev without triggering what some Ukrainians call "World War Three", Reuters says.

The White House said in a statement on the call with Putin that Obama "emphasized that Russia's actions were in violation of Ukraine's sovereignty and territorial integrity and that, in coordination with our European partners, we are prepared to impose additional costs on Russia for its actions".

The European Union will raise the stakes on Monday by slapping sanctions on officials. EU diplomats were haggling over a list of people in Crimea and Russia who will be hit with travel bans and asset freezes for actions which "threaten the territorial integrity, sovereignty and independence of Ukraine."

An initial list of 120 to 130 names will be whittled down to "tens or scores" before EU foreign ministers take the final decision in Brussels on Monday, diplomats said. Ministers are also expected to cancel an EU-Russia summit scheduled for June in Sochi, where Putin last month hosted the Winter Olympics.

The EU is working to revive a trade and aid deal with Ukraine which ousted president Viktor Yanukovich rejected in November in favor of cash from Moscow, triggering protests that led to bloodshed in Kiev and his flight to Russia last month.

The risk of Europe becoming locked in a damaging spiral of economic retaliation with Moscow, from which it buys much of its energy, depended on Russia, Dutch Foreign Minister Frans Timmermans said ahead of the EU meeting in Brussels: "I would do anything possible to avoid sanctions, because I believe everybody will suffer if we get into sanctions," he said, according to Reuters.

The U.S. administration is also preparing to identify Russians to punish with visa bans and asset freezes that Obama authorized this month. It, too, is likely to act on Monday.