The unrest in Ukraine is "the biggest crisis in Europe in the 21st century", British Foreign Secretary William Hague said on Monday, as he warned Russia to back down in Crimea.

"It's difficult to rank these things but it's certainly the biggest crisis in Europe in the 21st century and it will require all our diplomatic efforts," Hague told BBC radio from Kiev, where he is meeting the interim Ukrainian government.

The U.S. and Russian foreign ministers will attend French-sponsored talks in Paris Wednesday on helping Lebanon cope with the conflict in Syria, but the talks look set to be overshadowed by the Ukraine crisis, France said.

U.S. Secretary of State John Kerry was scheduled to come for the meeting of the International Support Group for Lebanon meeting but French Foreign Minister Laurent Fabius said Monday that Russia's Sergei Lavrov would also attend the conference.

Russia's central bank on Monday raised its main interest rate to 7.0 percent from 5.50 percent in a surprise move that came as concerns mounted over the consequences of intervention in Ukraine for the Russian economy.

The Bank Rossii (Bank of Russia) said the hike would take effect from 0700 GMT Monday and implied the decision was aimed at limiting the knock-on economic effects of the Ukraine crisis.

Ukraine's deposed president Viktor Yanukovych is still the legitimate head of state of the country even if his authority is "negligible", Russian Prime Minister Dmitry Medvedev said Monday.

Medvedev added that Russia does not recognize the new authorities who took power in Kiev after the overthrow of Yanukovych, saying they had violated the constitution.

U.S. Secretary of State John Kerry will travel to Kiev this week in a show of support for the embattled leadership, as Washington and its allies slammed Moscow for violating Ukraine's sovereignty.

The top U.S. diplomat led warnings from around the globe that Moscow risked being stripped of its coveted seat as one of the Group of Eight nations, and could face a damaging economic fallout for sending troops into Ukraine's Crimean Peninsula.

Russia's Vladimir Putin has agreed to a proposal from Angela Merkel to set up a contact group on Ukraine, the German government said Sunday.

The German chancellor put the proposal to Putin in a telephone conversation late Sunday in which she also "accused the Russian president of violating international law with the unacceptable Russian intervention in Crimea," said a government statement.

U.N. deputy chief Jan Eliasson will fly to Ukraine, where a threatened Russian invasion has risked sparking the worst crisis since the Cold War.

The world body's Secretary-General Ban Ki-moon ordered Eliasson to visit Ukraine after two emergency sessions of the U.N. Security Council that saw Western countries trade blame with Russia for the unfolding crisis in Ukraine.

The finance ministers of the world's top industrialized countries promised a firm aid package to Ukraine on Sunday, while calling for reform and a central role by the International Monetary Fund.

Earlier, leaders of the countries symbolically billed themselves as the "G7," saying Russia's actions were incompatible with the Group of Eight Nations, which Moscow joined in 1997, and withdrew from preparations for June's G8 summit in Sochi.

NATO allies on Sunday urged the deployment of international observers to Ukraine and said the alliance sought "to engage" with Moscow at NATO-Russia talks.

"We urge both parties to immediately seek a peaceful resolution through dialogue, through the dispatch of international observers under the auspices of the United Nations Security Council or the OSCE," said a statement issued after almost eight hours of talks between NATO's 28 ambassadors.

U.S. Secretary of State John Kerry bluntly warned Russia Sunday that it risked losing its seat among the prestigious Group of Eight nations, as well as economic turmoil and sanctions, over the Ukraine crisis.

The top U.S. diplomat hit the Sunday talk shows to ratchet up pressure on Moscow, after he convened a crisis conference call with European and Canadian foreign ministers and President Barack Obama held a 90-minute phone call with Russian counterpart Vladimir Putin Saturday.