Russia plans to reinforce its Black Sea Fleet based in Crimea, which it annexed from Ukraine in March, Defense Minister Sergei Shoigu said on Tuesday.

"This year, the Black Sea fleet will receive (new) submarines and new-generation ships," Shoigu said in televised comments, without giving details of numbers.

Russia on Tuesday rejected a new peace initiative for Ukraine as fears of open war mounted over the ex-Soviet republic, whose troops are waging an increasingly deadly offensive against pro-Moscow rebels.

"Chaos and the risk of civil war" are looming, French President Francois Hollande warned after Kiev announced the death toll from an assault on a rebel-held flashpoint town in the east had climbed over 34.

The death toll from a military offensive in a flashpoint town in east Ukraine rose to at least 34, officials said Tuesday, amid fresh warnings of civil war and the shutdown of a major airport in the region.

"Chaos and the risk of civil war" are now looming in Ukraine, said French President Francois Hollande, reflecting a frantic European push under way for a diplomatic solution before it is too late.

A leader of the Crimean Tatars, a Muslim minority group on the Moscow-controlled peninsula, said Monday that the new Russian authorities had threatened to prosecute them for "extremism".

The leader of the ethnic minority's assembly, known as the Mejlis, Refat Chubarov, said the chief regional prosecutor had on Sunday read him an official warning to stop "extremist activities".

The United States is sending a top Treasury official to Germany, France and Britain this week to discuss sanctions against Russia over the Ukraine crisis.

David Cohen, the Treasury Department's under secretary for terrorism and financial intelligence, will discuss issues related to illicit finance and sanctions that have been imposed "in response to Russia's illegal action in Ukraine," the department said Monday in a statement. 

Russian President Vladimir Putin on Monday signed new legislation introducing harsh punishments for the justification or denial of Nazi war crimes.

The legislation makes it a criminal offence to deny facts established by the Nuremberg trials regarding the crimes of the Axis powers and to disseminate "false information about Soviet actions" during World War II.

International security body the OSCE is emerging as an important player in the Ukraine crisis, with both the West and Moscow calling for it to perform an even bigger role.

Herewith are some facts about the Organization for Security and Cooperation in Europe and its current activities in Ukraine.

Russia warned Monday that Europe's peace was at risk over the escalating crisis in Ukraine, where deadly fighting raged around a flashpoint eastern city and the interim president warned of "war."

European leaders fearing all-out civil war was breaking out on their eastern flank have launched a desperate new peace bid, trying to force Ukraine and Russia to find a negotiated solution before it is too late.

The head of the Organization for Security and Cooperation in Europe, Swiss President Didier Burkhalter, will visit Moscow on Wednesday for talks on the crisis in Ukraine, the Kremlin said.

Burkhalter will fly to Moscow on May 7, the Kremlin said in a statement after President Vladimir Putin spoke by phone with German Chancellor Angela Merkel.

Several dozen Russian planes including what appeared to be strategic bombers and fighter jets have been spotted in the sky above the Moscow-controlled peninsula of Crimea, witnesses and experts said.

According to Russian media, President Vladimir Putin is poised to visit Crimea on Friday after overseeing the main military parade on Red Square when Russia celebrates its victory over Nazi Germany in World War II.