U.S. President Barack Obama will visit Poland in June as part of a European tour that will also include stops in France and Belgium, Vice President Joe Biden announced Wednesday.

The visit to Poland comes as Washington tries to allay the concerns of its allies over the escalating crisis in neighboring Ukraine. 

A leader of the self-declared Donetsk People's Republic said Wednesday the eastern Ukrainian region will not take part in presidential polls as he traveled to Russia on an apparent mission to seek support.

"Presidential polls have been called by the illegitimate authorities and therefore they are illegitimate," Denis Pushilin, one of the leaders of the self-declared Donetsk Republic, told reporters in Moscow.

Four Danish F-16 warplanes landed in Estonia on Wednesday, marking the first time NATO planes will be stationed in the ex-Soviet state and Russian neighbor. 

The aircraft and a supporting team of 60 people arrived at the Amari air base in the west of the Baltic state at a time when NATO is reinforcing its presence in the region to allay concerns triggered by the Ukraine crisis.

The European Union is considering beefing up sanctions against Russia by targeting President Vladimir Putin's inner circle but some member states are "very reluctant," EU diplomats said Wednesday.

A proposal seen by Agence France Presse that was circulated to member states by Britain this week calls on the bloc to increase pressure on Putin by sanctioning his "inner circle" or "money men" in response to the deepening crisis in Ukraine.

Ukraine's army is probing rebel defenses around the besieged eastern town of Slavyansk, where a long stalemate is under way in what has become a crucible for the worsening crisis.

Three outlying checkpoints set up by insurgents around the town of 160,000 inhabitants were "liquidated" overnight with no casualties on either side, the interior ministry in Kiev said in a statement.

Pro-Russian insurgents Wednesday lifted an armed siege of a regional police headquarters in the eastern Ukrainian city of Lugansk after the chief officer agreed to resign, as Kiev said its army is on "full combat alert" against a possible Russian invasion.

A police spokeswoman said the officers inside had refused to give up their weapons to a crowd of some 1,000 pro-Moscow militants led by 30 or so armed men carrying Kalashnikov assault rifles and grenade-launchers, who had tried to storm the building overnight.

Russia and the United States stepped up their rhetoric over the spiraling crisis in Ukraine, as pro-Moscow militants shored up control of key buildings in the country's increasingly chaotic east Wednesday.

President Vladimir Putin threatened that U.S. sanctions against Moscow could harm Western energy interests in Russia, which the West blames for stoking the worst confrontation since the end of the Cold War.

The United States and Britain said Tuesday they were determined to track down billions of dollars of Ukrainian assets allegedly looted under the regime of deposed president Viktor Yanukovych.

U.S. Attorney General Eric Holder and British Home Secretary Theresa May told an international conference on asset recovery that those responsible would be held accountable.

More than 3,000 pro-Russian protesters stormed the regional administration building in the eastern Ukraine city of Lugansk on Tuesday, as Washington described recent violence in the region as “terrorism.”

A vanguard of around 20 youths armed with metal bars broke a window to get inside the building, which was not protected by police.

If Russia invades Ukraine, "they don't need tanks, they can just walk," smiles Lev Nikolaevich, pointing with his deformed finger to the Russian border just behind the trees. "And we will welcome them."

While the world frets about an invasion amid the worst East-West confrontation since the Cold War, the 90 or so people in the tiny border village of Hryhorivka are more relaxed.