U.S. President Barack Obama raised the stakes in an East-West confrontation over Crimea on Thursday, March 20 by targeting some of Russian President Vladimir Putin's closest long-time political and business allies with personal sanctions, Reuters said.

The extension of visa bans and asset freezes into Putin's inner circle came as Moscow rushed to consolidate the annexation of the Black Sea peninsula, and to boost its military presence in the region.

Russian troops took over three Ukrainian warships in Crimea on Thursday, using stun grenades in one incident, a Ukrainian spokesman said. Kyiv also said it had begun withdrawing its border guards, surrounded and outnumbered by Russian forces, from Crimea to the mainland.

The 20 names added to the U.S. blacklist included Kremlin banker Yuri Kovalchuk and his Bank Rossiya, major oil and commodities trader Gennady Timchenko and the brothers Arkady and Boris Rotenberg, linked to big contracts on gas pipelines and at the Sochi Olympics, as well as Putin's chief of staff and his deputy, the head of military intelligence and a railways chief.

Most grew rich after being associated with Putin since the former KGB officer began his ascent to power in the mayor's office of St Petersburg in the 1990s.

In a statement explaining the sanctions, the U.S. Treasury said: "Gennady Timchenko is one of the founders of Gunvor, one of the world's largest independent commodity trading companies involved in the oil and energy markets.

"Timchenko's activities in the energy sector have been directly linked to Putin. Putin has investments in Gunvor and may have access to Gunvor funds."

Putin has denied any link with Gunvor in the past. The Swiss-based oil trading company said in a statement that Putin had no ownership of Gunvor and "any understanding otherwise is fundamentally misinformed and outrageous".

It also said Timchenko, who has Finnish as well as Russian citizenship, had sold his 43 percent stake in Gunvor to its chief executive, Torbjorn Tornqvist, on Wednesday as part of what the company called a "contingency plan".

Moscow retaliated by announcing its own sanctions against senior U.S. politicians, with the Foreign Ministry saying U.S. action would "hit the United States like a boomerang".

European Union leaders meeting in Brussels agreed to add 12 more Russian and Crimean officials to their sanctions list for visa bans and asset freezes and to cancel a planned EU-Russia summit in Sochi and forthcoming bilateral summits with Moscow.

The names will be published on Friday but diplomats said the EU would not go as far as Washington in hitting Putin's money men. German Chancellor Angela Merkel said the EU had a different legal basis and needed proof that individuals had been directly involved in violating Ukraine's sovereignty.

She told reporters the executive European Commission had been tasked to prepare for possible broad economic sanctions in case Russia moved further into Ukraine or acted to destabilize the situation. Officials said the sanctions could affect trade, finance, energy and arms supplies.

The EU also agreed to send its own observer mission to Ukraine if Moscow continues to block monitors from the pan-European Organisation for Security and Cooperation in Europe.

EU sources said behind closed doors the leaders also discussed a radical plan to reduce their dependence on Russian energy by agreeing to negotiate gas purchases collectively with Moscow instead of country-by-country. The leaders made no mention of this at a late-night news conferences.

The sources said the EU would also accelerate work to upgrade cross-border energy networks to reduce individual member states' vulnerability to supply cuts, and speed up building new liquefied natural gas import terminals to diversify suppliers.