NATO said Thursday it had yet to see "any indications" that Russia has withdrawn its troops from the Ukraine border, a day after President Vladimir Putin claimed to have done so.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

Russian President Vladimir Putin told rebels in Ukraine to halt plans for independence votes and said his troops have pulled back from the border, but his apparent change of heart received short shrift from Kiev and Washington.

Putin on Wednesday also hailed a planned May 25 presidential election in Ukraine -- previously condemned by the Kremlin -- as a "move in the right direction".

Russia is deploying covert fighters and "enormous propaganda" to prevent Ukraine holding a presidential election later this month, British Foreign Secretary William Hague said on Wednesday during a visit to Kiev.

If Moscow succeeds, it would be "a terrible blow to democracy," said Hague, who accused Russia of engineering an independence referendum in Ukraine's east this weekend to undermine the May 25 presidential poll.

Ukrainian presidential candidate and self-made billionaire Petro Poroshenko defended the deployment of security forces against pro-Russian rebels, during a visit to Germany on Wednesday.

"For terrorists we should find the language they understand and that is force," he told reporters, after talks with a senior member of Chancellor Angela Merkel's conservatives.

Pro-Russian rebels on Wednesday fought troops to try to regain strategic positions in east Ukraine, while the West threw its weight behind a last-ditch diplomatic bid to calm the crisis ahead of a crucial presidential election.

The head of the OSCE was in Moscow to appeal to Russian President Vladimir Putin to help calm hostilities in the former Soviet republic.

Russia is gearing up for hugely patriotic celebrations of its victory over Nazi Germany in World War II on Friday, but festivities in neighboring Ukraine will be muted amid fears of provocations.

The May 9 commemorations come at an extremely sensitive time for the two Slavic nations that fought side by side against Nazi Germany but are now locked in an unprecedented confrontation that threatens to tip Ukraine into civil war.

U.S. senators were split Tuesday over when to impose new sanctions on Russia, with Republicans saying President Barack Obama should not wait until Ukraine's upcoming election before slapping Moscow with sector-wide penalties.

The White House has said it remains prepared to impose biting new sanctions on Russia, which has annexed the Crimean peninsula and furthered its aggression in unrest-plagued eastern Ukraine, should its forces disrupt Ukraine's May 25 poll.

German Chancellor Angela Merkel said Tuesday it would be a "pity" if the Russian president were to "use" the commemoration of World War II victory for visiting Crimea.

Merkel was responding to a reporter's question about media reports that Vladimir Putin could attend a May 9 military parade in his first visit to the peninsula since Russia annexed the territory in March.