The unrest in Ukraine has culminated in the worst violence since the toppling of pro-Russian Viktor Yanukovich from the presidency in February.
More than 40 people were killed in Odessa on Friday, most caught in a trade union building set on fire after pro-Russian separatists and "Unified Ukraine" demonstrators clashed in the southern port city.
The violence broke out on Friday afternoon, when the two sides confronted each other, to outnumbered police who were unable to control crowds throwing stones, bottles and stun grenades.
Later on Friday, a fire was started by unidentified men in the government building where separatists were sheltering.
Interfax Ukraine news agency said 10 fire engines were brought on the scene to put out the flames and activists from both sides tried to rescue people trapped in the burning building.
Police said 30 people choked to death on smoke and eight others jumped out of windows of the burning building.
Earlier on Friday, Ukraine's military had attempted an assault against pro-Russian armed men in the eastern city of Slovyansk that ended with two of the military helicopters being shot down by the rebels.
The incident left at least three pro-Moscow rebels, two Ukrainian military servicemen and two civilians dead, according to information provided by Ukraine's defence ministry and the rebel spokeswoman.
Ukraine's defence ministry later said it had arrested four men suspected of shooting down the helicopters.
Vyacheslav Ponomarev, the self-proclaimed mayor of Slovyansk, urged all men in the pro-Russian stronghold to join efforts to repel the assault by Ukrainian security forces.
Events in Slovyansk prompted Russia to call for an emergency meeting of the United Nations Security Council, ITAR-TASS news agency reported on Friday.
Meanwhile, President Barack Obama and German Chancellor Angela Merkel warned Russia it would face additional sanctions against key sectors of its economy if Moscow disrupted Ukraine's plan to hold elections on May 25.
The two leaders made the threat when they addressed a joint news conference at the White House, after Oval Office talks were dominated by the situation in Ukraine.