The East-West split re-opened by the crisis in Ukraine hardened Wednesday when President Barack Obama threw Washington's weight firmly behind Ukraine in its stand-off with Moscow.

Obama welcomed Kiev's interim premier Arseniy Yatsenyuk to the White House and stood by his side as both leaders sternly warned Russia that Ukraine would not surrender its sovereignty.

Belarussian President Alexander Lukashenko said Wednesday he would ask Russia to station up to 15 jets in his country in response to NATO's moves over the Ukraine crisis.

"If NATO decided, together with the Americans, to ramp up their air force presence near our borders, what, should we just watch them?" the maverick leader said, speaking at a meeting of his security council.

Hundreds of people have fled Crimea and the situation could deteriorate further as tensions continue to rise on the Russian-controlled peninsula, Ukraine's national security chief warned Wednesday.

"Today there are hundreds of them, but if separatists continue their activities in the Crimea, the number of refugees may increase," National Security and Defense Council Secretary Andriy Parubiy told reporters.

The Group of Seven most developed economies on Wednesday said a Moscow-backed referendum in Crimea on switching over to Kremlin rule would have "no legal effect" and called on Russia to back down.

The G7 -- Britain, Canada, France, Germany, Italy, Japan and the United States -- along with European Union leaders called on Russia "to cease all efforts to change the status of Crimea contrary to Ukrainian law and in violation of international law," according to a statement released by the White House.

The U.S. Congress on Tuesday demanded Russia immediately withdraw from Ukraine and urged U.S. President Barack Obama to impose punitive economic sanctions on Moscow.

The Senate expressed its support for Ukraine by passing strongly worded resolutions, using tough language against Russia and urging it be suspended from the Group of 8 world powers.

President Vladimir Putin has seen his approval rating climb in Russia due to his strong stance on military intervention in Ukraine, several opinion polls show.

More than two out of three Russians (69 percent of those interviewed), say they back Putin's actions, a poll by the independent Levada agency found in late February after interviewing 1,603 people in 45 regions.

Ukraine's former prime minister, Yulia Tymoshenko, has experienced some relief from debilitating back pain since arriving in Germany for care last week, the hospital treating her said Tuesday.

Tymoshenko, a leader of the pro-Western Orange Revolution in 2004, suffers from herniated discs and underwent medical examinations at Berlin's renowned Charite university hospital after landing on Saturday.

Switzerland, which led a push to block assets of ousted Ukrainian leader Viktor Yanukovych and others, has frozen the holdings of nine more individuals including one of his sons.

Swiss authorities, which last month ordered a freeze on the assets of Yanukovych, his coal magnate son Oleksandr and 18 other Ukrainians, published an order late on Monday extending their net.

Pro-Moscow lawmakers in Crimea voted for independence from Ukraine on Tuesday in a precursor to a referendum this weekend for the region to become part of Russia.

The local assembly approved a "declaration on the independence of the autonomous republic of Crimea and the city of Sevastopol" with 78 out of 81 lawmakers present voting in favor.

Ousted Ukrainian leader Viktor Yanukovych on Tuesday said that he was still the country's legitimate president and commander-in-chief, predicting he would be able to return to Kiev soon.

"I remain not just the sole legitimate president of Ukraine but also commander-in-chief," he said in his first public appearance in over a week, in the southern Russian city of Rostov-on-Don.