KIEV - Ukrainian security forces killed about 20 pro-Russian rebels who tried to seize control of police headquarters in the eastern port city of Mariupol on Friday, the Interior Minister said.
Minister Arsen Avakov said the attempt by "terrorists" to storm the building in a key industrial and shipping center turned into a pitched battle within its walls. Some rebels had fled and were seeking shelter in the town.
Meanwhile on Friday, Russian President Vladimir Putin flew in to Crimea for parades marking the Soviet victory in World War Two, his first visit since annexing the peninsula from a Ukraine that Russia says has been taken over by fascists.
Pro-Moscow rebels in east Ukraine have planned a referendum on Sunday to follow Crimea in breaking from Kiev.
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 a "provocation" that was intended deliberately to escalate the crisis.
Earlier on Friday, Putin 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.
He was expected to attend a military parade and other war anniversary events in Crimea. This year is also the 70th anniversary of the battle in which the Red Army won back control of the Black Sea peninsula from the Nazis.
Putin is sure to receive a hero's welcome in the port of Sevastopol, where Russia's Black Sea Fleet is based.
But NATO Secretary-General Anders Fogh Rasmussen said: "His visit to Crimea is inappropriate."
The head of the US-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.
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.