Russia on Tuesday banned the leader of Crimea's pro-Kiev Tatar community from entering the Black Sea peninsula for five years, the Tatar assembly said.

Mustafa Dzhemilev was handed an official order barring him from returning to Crimea as he crossed to mainland Ukraine from the territory that Moscow controversially annexed last month, the assembly said in a statement.

U.S. Vice President Joe Biden on Tuesday accused Russia of "trying to pull Ukraine apart" and pledged Washington's strong support for Kiev's leaders, as a Cold War-style confrontation over the former Soviet republic ratcheted up.

Biden was speaking in Kiev amid worrying signs on the ground that diplomacy was failing to calm the crisis.

Russian Foreign Minister Sergei Lavrov and his U.S. counterpart John Kerry urged each other in a telephone conversation Monday to use their influence to get Ukraine's rival sides to honor last week's Geneva accord.

Kerry told Lavrov that "concrete steps" towards defusing the Ukraine crisis should include "publicly calling on separatists to vacate illegal buildings and checkpoints, accept amnesty and address their grievances politically," State Department spokeswoman Jen Psaki said.

A man armed with a hunting rifle who took three people hostage in a bank in southwestern Russia on Monday has surrendered, police said.

"The talks have been very successful," regional police spokesman Yevgeny Kamalov told Agence France Presse, adding that the hostages had now been released.

New laws passed on Monday in Russia make it easier for native speakers and those who can prove they or their families have lived within the borders of the former Russian empire or Soviet Union to get citizenship.

The amendments were signed into law by President Vladimir Putin, who annexed the Russian-speaking peninsula of Crimea in Ukraine last month and has asserted his right to protect Russian speakers across the former Soviet bloc.

Russian President Vladimir Putin said on Monday he had signed a decree rehabilitating Crimea's Tatars, an ethnic group accused of collaborating with Nazi Germany and exiled under Stalin.

"I have signed a decree to rehabilitate the Crimean Tatar population of Crimea, the Armenian population, Germans, Greeks, all those who suffered during Stalin's purges," Putin told a government meeting.

Turkey has agreed "in principle" to increase shipments of natural gas from Russia via the Blue Stream pipeline, its energy minister said on Monday. 

The two countries will increase capacity through the pipeline, which crosses the Black Sea, from 16 billion cubic metres annually to 19 billion cubic metres, said Energy Minister Taner Yildiz. 

Vice President Joe Biden started a two-day visit to Kiev on Monday in a pointed show of U.S. backing as Russia accused Ukraine's government of "grossly breaching" a deal designed to de-escalate separatist tensions.

Biden was to reinforce a message to Russia -- which Washington sees as supporting Ukraine's insurgency -- that time is running out for it to persuade pro-Kremlin rebels holding a string of eastern towns to comply with the pact struck in Geneva on Thursday, a senior U.S. official told reporters travelling with the vice president.

Ukrainian Prime Minister Arseniy Yatsenyuk warned in an interview that aired Sunday that Russia is undermining global stability and nuclear nonproliferation efforts amid an ongoing crisis between Kiev and the Kremlin.

Ahead of a visit by U.S. Vice President Joe Biden, he also called for financial and economic support and help modernizing Ukraine's military -- while stopping short of asking for weapons.

A Kremlin spokesman on Sunday dismissed as "absurd" claims that Washington could target President Vladimir Putin directly if it imposes further sanctions against Russia.

Quoting anonymous sources, The Times said in an article published Friday that the United States was looking at imposing sanctions on Putin, who is estimated to hold some $40 billion in Swiss accounts.