Election preparations were underway in east Ukraine on Saturday on the eve of an independence vote called by pro-Russian separatists as government forces pushed ahead with a military offensive against the rebels.
The voting comes after Russian President Vladimir Putin visited Crimea on Friday, for the first time since its annexation by Moscow, as the fighting in eastern Ukraine left more than 20 dead.
Despite a surprise call from Putin this week to delay the independence referendums, rebels holed up in more than a dozen towns and cities in eastern Ukraine vowed to press ahead with votes that are bound to increase tensions.
Amid the military operation by the Ukrainian army to oust rebels in the region that has left dozens dead, Sunday's referendum asks people if the industrial region of Donetsk should become independent from Kiev and is seen as a potential stepping stone by some towards joining Russia.
A similar vote is also set to be held in the neighbouring Lugansk region.
Together, the two regions have a total population of 7.3 million, out of 46 million for all of Ukraine.
The head of Donetsk's separatists, Denis Pushilin, has said the referendum will be held in “90 percent of the towns in the region” and turnout is expected to be 60 percent.
The determination to hold the vote, despite Putin's call for a postponement, dashed hopes of easing the crisis.
Putin's Crimea visit further spiked the tensions and drew a sharp rebuke from authorities in Kiev, who accused the Russian strongman of stoking tensions with his visit to Sevastopol, home to Russia's Black Sea fleet.
“This provocation once again confirms that Russia deliberately seeks further escalation of tensions,” the foreign ministry said, calling the visit a “flagrant violation of Ukraine's sovereignty”.
The White House also condemned the trip, with National Security Council spokeswoman Laura Magnuson saying it “will only serve to fuel tensions”.
NATO head Anders Fogh Rasmussen called the visit “inappropriate” given Crimea's “illegal, illegitimate” annexation.
A poll released Thursday by the Pew Research Centre in the United States though, suggested that 70 percent of Ukrainians in the east want to stay in a united country, while only 18 percent back secession. Two in three respondents in the east, however, are unhappy with the Western-backed government in Kiev.
With unease high ahead of the independence vote, fighting between Ukrainian troops and pro-Moscow militants erupted in the southeastern port city of Mariupol.
An attempt by around 60 rebels armed with automatic weapons to storm the city's police headquarters turned into a “full-scale military clash” when army and interior ministry troop reinforcements arrived, Interior Minister Arsen Avakov said on his official Facebook page.
He said the death toll from the near-two-hour combat stood at 20 rebels and one policeman, while another four policemen were wounded and four rebels were captured.
Witnesses in Mariupol told AFP the fighting was ferocious and involved an exchange of automatic gunfire and shelling from eight armoured vehicles.
The police headquarters was gutted by fire and, after the battle, firemen were at the scene trying to extinguish the flames.
Russia's annexation of Crimea in March set off the worst diplomatic crisis in the West's relations with Moscow since the end of the Cold War.
It has been followed by uprisings and fighting in eastern Ukraine that have raised concerns of a civil war erupting on Europe's doorstep.
Putin flew to Sevastopol after overseeing the traditional Victory Day parade in Moscow's Red Square marking the Soviet victory over the Nazis in World War II.
Addressing about 11 000 troops who marched alongside tanks, armoured vehicles and mobile missile systems, Putin hailed Russia's “all-conquering patriotic force”.
In contrast to the display of military hardware on Red Square, Ukraine held muted Victory Day celebrations in a bid to avoid violence.
Previous violence in Ukraine in recent weeks saw 14 troops killed, three helicopter gunships downed and 66 servicemen wounded in assaults on the rebels. The fighting also claimed the lives of more than 30 insurgents.
Clashes that resulted in a horrific inferno in the southern port city of Odessa last week claimed another 42 lives, most of them pro-Russian activists.
Unrest was also reported Friday in the eastern city of Donetsk, with pro-Russian militants saying two of their number were wounded by brief gunfire from Ukrainian troops stationed at a sanatorium on the outskirts of the city. The troops withdrew from the area after talks, they said.
The Ukrainian prosecutor's office said it was also investigating the death of an Orthodox priest allegedly shot eight times at a rebel checkpoint in the Donetsk region on Thursday.
The crisis in Ukraine kicked off after the ouster of the country's pro-Kremlin president Viktor Yanukovych in February and the West is counting on a presidential election on May 25 to stem the chaos.
The violence has prompted many Western politicians to warn that the country of 46 million people is slipping towards a civil war.
In a phone call with US Secretary of State John Kerry on Friday, Russian Foreign Minister Sergei Lavrov stressed the need for the “rapid launch” of talks between Kiev and regional authorities in eastern Ukraine.
The Russian foreign ministry said in a statement that Moscow urged Washington to “work with Kiev to achieve the end of military operations in the southeast (of Ukraine), the release of political prisoners and amnesty for protesters”.
Accusing Russia of backing pro-Moscow militants, the United States and European Union have imposed sanctions on Putin's inner circle. EU ministers are to meet on Monday to consider further measures. - Sapa-AFP