Fierce fighting between Ukrainian troops and pro-Russian rebels broke out in the southeastern city of Mariupol on Friday, killing 21 people, all but one of them insurgents, Ukraine's interior minister said.

An attempt by around 60 rebels armed with automatic weapons to storm the police headquarters turned into a "full-scale military clash" when army and interior ministry troop reinforcements arrived, Arsen Avakov said on his official Facebook page.

President Vladimir Putin reviewed Russian ships in the port of Sevastopol on Friday as he visited Russia's naval base in the Crimean port for the first time since Moscow's annexation of the Black Sea peninsula.

Russian television showed live footage of Putin standing on a white vessel passing at least 10 navy ships in the port, stopping at each to congratulate the sailors for Victory Day, Russia's celebration of the defeat of Nazi Germany.

Thousands of Russian troops are marching on Red Square in the annual Victory Day parade in a proud display of the nation's military might amid the escalating tensions over Ukraine.

President Vladimir Putin made no reference to the situation in Ukraine when he opened Friday's parade, focusing on the historic importance of the victory over Nazi Germany.

U.S. President Barack Obama has no plans to meet one-on-one with his Russian counterpart Vladimir Putin in June when both attend the D-Day anniversary in France, the White House said Thursday.

Russia's intervention in the political crisis in Ukraine has triggered the worst chill in relations between Washington and the Moscow since the end of the Cold War.

The spiritual leader of Crimea's Tatars appealed to the international community Thursday not to forget the Black Sea peninsula, annexed by Russia, as attention has turned to growing violence in eastern Ukraine.

"We are concerned that because of Russia's recent actions, the issue of Russia's occupation of Crimea has become secondary in light of the potential threat of Russian forces invading across the eastern border of Ukraine," Mustafa Dzhemilev, spiritual leader of the ethnic Tatar community in Crimea, said in Vienna.

A U.S. warship arrived on Thursday in Georgia's port of Batumi, the U.S. embassy said, sending a message of support to NATO allies amid the spiraling crisis in neighboring Ukraine.

The USS Taylor's "presence in Georgia reaffirms the United States' commitment to strengthening ties with NATO allies and partners like Georgia, while working toward mutual goals of promoting peace and stability in the region," the embassy said in a statement.

Russian President Vladimir Putin will attend ceremonies in France next month marking the 70th anniversary of D-Day despite the Ukraine crisis, the ambassador to Paris said Thursday.

"President Putin has been invited by President (Francois) Hollande to take part in the ceremonies to commemorate the Normandy landings on June 6. He has accepted the invitation. He will be coming on June 6," Alexander Orlov told BFM television.

NATO said Thursday it had yet to see "any indications" that Russia has withdrawn its troops from the Ukraine border, a day after President Vladimir Putin claimed to have done so.

"Let me assure you that if we get visible evidence that they are actually pulling back their troops, I will be the very first to welcome it," NATO Secretary General Anders Fogh Rasmussen said in Warsaw.

Russia on Thursday test-launched several ballistic missiles during planned exercises overseen by President Vladimir Putin, news reports said, as a crisis raged in neighboring Ukraine.

The Russian military fired a Topol intercontinental ballistic missile from its northern test site in Plesetsk, as well as "several" shorter-range missiles from its submarines in the Northern and Pacific Fleets, news agencies cited defense officials as saying.

Pro-Moscow rebels fighting in east Ukraine vowed Thursday to press on with disputed independence referendums, defying a call from President Vladimir Putin to postpone the vote in a bid to ease tensions.

"The vote will happen on May 11," the leader of the self-proclaimed People's Republic of Donetsk, Denis Pushilin, declared to reporters.