President Vladimir Putin flew in to Crimea on Friday, marking the Soviet victory in World War Two and proclaiming the success of the peninsula's seizure from a Ukraine that Russia says has been taken over by fascists.

In east Ukraine, where pro-Moscow rebels plan a referendum on Sunday to follow Crimea in breaking from Kiev, between three and 20 people were reported killed in the port of Mariupol, one of the biggest clashes yet between Ukrainian forces and separatists.

The United States criticized Putin's visit to Crimea on Friday as provocative and repeated the U.S. rejection of Moscow's annexation of the Black Sea peninsula.

"This trip is provocative and unnecessary. Crimea belongs to Ukraine and we don't recognize, of course, the illegal and illegitimate steps by Russia in that regard," State Department spokeswoman Jen Psaki told reporters at her daily briefing.

The United States has not seen a withdrawal of Russian forces from its border with Ukraine announced by Putin earlier this week, the spokeswoman said, urging Moscow to calm tensions.

"We want Russia to take de-escalatory steps. We believe they can influence the actions of separatists on the ground. There is more they can do. And, obviously, President Putin's trip to Crimea is not in the direction we are looking for," she said.

The head of NATO, locked in its gravest confrontation with Russia since the Cold War, condemned Putin's visit to Crimea, whose annexation in March has not been recognized by Western powers. He also renewed doubts over an assurance by the Kremlin leader that he had pulled back troops from the Ukrainian border.

The government in Kiev called Putin's visit, his first since the takeover of the region two months ago, a "provocation" that was intended deliberately to escalate the crisis.

Watching a military parade in Sevastopol on the Black Sea, Putin said: "I am sure that 2014 will go into the annals of our whole country as the year when the nations living here firmly decided to be together with Russia, affirming fidelity to the historical truth and the memory of our ancestors."

Earlier in the day, he had presided over the biggest Victory Day parade in Moscow for years. The passing tanks, aircraft and intercontinental ballistic missiles were a reminder to the world - and Russian voters - of Putin's determination to revive Moscow's global power, 23 years after the Soviet collapse.

"The iron will of the Soviet people, their fearlessness and stamina saved Europe from slavery," Putin said in a speech to the military and war veterans gathered on Red Square.

But NATO Secretary-General Anders Fogh Rasmussen said: "His visit to Crimea is inappropriate."

The head of the U.S.-led defense pact was speaking in formerly Soviet Estonia, one of a host of east European nations that joined after the collapse of communism, seeking refuge from the power of Moscow, which many in the region regarded as having enslaved them following its victory in World War Two.

The European Union also criticized Putin, saying he should not use the commemoration of victory in World War Two to showcase Russia's annexation of the region.

"The European Union notes with regret the presence of President Vladimir Putin at a military parade ... in Sebastopol, Crimea," Maja Kocijancic, spokeswoman for EU foreign policy chief Catherine Ashton, said.

Ukrainian Prime Minister Arseny Yatseniuk, in office since an uprising overthrew the Kremlin-backed elected president in Kiev in February, rejects Russian allegations that his power is the result of coup backed by neo-Nazi Ukrainian nationalists.

"Sixty-nine years ago, we, together with Russia, fought against fascism and won," he said after a Victory Day church service in the capital. Now, he added, "history is repeating itself but in a different form."

Where Russia and Ukraine stood shoulder to shoulder in the past against Germany, now Germany was "standing shoulder to shoulder with us," along with the United States and Britain.

Ukraine's SBU security service accused Russian saboteurs of setting a fire that briefly disrupted state broadcasting services and the Foreign Ministry issued a statement describing Putin's visit as a deliberate escalation of the crisis.

FIGHTING, BLAZE

In Mariupol, the region's main port on the Sea of Azov, Ukrainian Interior Minister Arsen Avakov said about 20 "terrorists" were killed when pro-Russian militants tried to seize the city's police headquarters.

That was a higher toll and somewhat different account from others. A member of Ukraine's parliament said Ukrainian forces had attacked the police headquarters in an attempt to drive out pro-Russian militants, and the building caught fire.

Oleh Lyashko said eight rebels had been killed in fighting.

But Donetsk medical authorities said three had been killed in fighting and 25 wounded. On Tuesday, Avakov said that more than 30 rebels had been killed in fighting near another eastern city, though that figure was not subsequently confirmed.

A local photographer in Mariupol told Reuters the building was ablaze and that two bodies were lying in the street outside.

"One of them is definitely a police officer," he said.

A fire a week ago during clashes in the southwestern port of Odessa killed dozens of pro-Russian activists, hardening attitudes in the east against the Kiev authorities.

Tetyana Ignatchenko, a journalist in Mariupol, said there was fierce fighting outside the police headquarters.

Ukrainian forces brought several armoured vehicles into the town, a major industrial centre with a population of about half a million but later withdrew.

Local media said separatists had seized a tank in the centre of town and built barricades around the city.

Russian Foreign Minister Sergei Lavrov said after a telephone call with U.S. Secretary of State John Kerry on Friday that Moscow hoped Washington would work with Kiev to end Ukraine's military operation against the separatists.

In Sevastopol, where Russia's Black Sea Fleet previously had to lease its base from Ukraine, servicemen and veterans marched in a parade before Putin's arrival that also included armored vehicles and anti-aircraft missiles. Banners read "Sevastopol without Fascists" and "It's our duty to remember."

"I'm here to prevent any provocations from the fascists. I served in a self-defense unit during March, and I consider it my duty to be here," said Natalya Malyarchuk, 52.

The ethnic Russian majority among Crimea's two million population broadly welcomed the Russian takeover that came in the wake of the Kiev uprising. Given by Soviet leaders to Ukraine only in the 1950s, many Russians long saw it as rightfully theirs. Western powers have imposed sanctions against Russia in response, but reactions have been muted.

Moscow says it has no direct control over the armed rebels in eastern Ukraine running Sunday's referendum on secession from Kiev for the mainly Russian-speaking region.

The European Union is likely to strengthen its targeted sanctions against Russia on Monday, Janusz Lewandowski, a member of the EU's governing commission, said on Friday. Diplomats said they would target about 15 people and several Crimean branches of Ukrainian companies taken over by Russians.

While Putin's redrawing of European borders has sparked great alarm across the continent, U.S. and European leaders are concerned not to harm their own economies by isolating Moscow.

And there is little popular support in the West for an armed conflict with Russia on behalf of Ukraine, a country that is not a member of NATO and where successive leaders have left a legacy of corruption, poverty and feeble state institutions.

In Sevastopol, factory worker Vasily Topol, 31, wearing a white T-shirt with an image of Putin in sunglasses and the words "Russia's Army", said life was better since becoming Russian.

"We have the greatest admiration for Putin, we are morally and materially better off since Crimea became part of Russia," he said, speaking on an embankment overlooking Russian warships.

In Slaviansk, the military stronghold of the separatists in eastern Ukraine, separatist "people's mayor" Vyacheslav Ponomaryov and a guard of militiamen led a march of around 2,000 people to lay flowers at a memorial to the World War Two dead.

Veteran Anatoly Strizhakov said: "Look at all these people - the children, the women, the pensioners... Today shows we've got the spirit to stand up to whatever the Ukrainians are planning."

Ponomaryov, who fired a pistol three times in the air during the ceremony, reassured people it would be safe to vote on Sunday. Voters in the two regions, with a combined population of over 6 million, will be asked to vote Yes to the secession of self-styled "People's Republics" in Donetsk and Luhansk.

Opinion polls in recent months have indicated that support for such a move is far from solid and it is unclear how many people will actually take part in voting. A referendum in Crimea in March, which many boycotted, backed secession by 97 percent.