The
United States has condemned the "senseless" violence that left at least
31 people dead in the south-west of Ukraine on Friday.
A blaze engulfed the building after running battles between pro-Kiev groups and separatists in the city.
Officials said some people were overwhelmed by smoke and others died after they jumped from the building.
"The violence and mayhem that led to so many senseless deaths and injuries is unacceptable," the state department said in a statement.
It also called on the Ukrainian authorities "to bring all those responsible to justice."
Investigation underway
Ukraine's interior ministry said the fire broke out in Odessa's Trade Unions House but it did not give details of how the blaze started.
The exact sequence of events is still unclear, but reports suggest the separatists had barricaded themselves inside the building and both sides were throwing petrol bombs.
Ukrainian Deputy Foreign Minister Danylo Lubkivsky told the BBC that an investigation was under way into the events but insisted Russia was behind the violence.
"The situation remains under control. But the security situation is threatened by Russian special agents," he said.
"That's why we are requesting and demanding Russian authorities to stop their efforts that can undermine regional security and stability."
Earlier on Friday four people were killed in clashes between the pro-Russian and pro-Kiev protesters in Odessa - the first such eruption of violence in the south after weeks of mounting unrest in Ukraine's east.
The BBC's David Stern in Kiev says the death toll in Odessa is the highest since violence broke out between pro-EU protesters and police in the capital in February.
The Black Sea city is ethnically mixed with a large number of Russian speakers, our correspondent says.
'A criminal adventure' The US called for the commitments made in Geneva last month to be implemented, referring to the agreement signed by all sides to dissolve illegal militia operating in eastern Ukraine.
On Friday President Oleksandr Turchynov said that Ukrainian armed forces killed many separatists in an offensive in the eastern city of Sloviansk.
During the day government forces took over pro-Russian checkpoints outside the city but separatists shot down two Ukrainian army helicopters, killing a pilot and another serviceman.
Russia responded by calling a meeting of the Security Council in New York, where it criticised Kiev and labelled the government offensive in eastern Ukraine a "criminal adventure."
Ukrainian government forces reportedly continued operations against separatists in the east at dawn on Saturday near the town of Kramatorsk.
Reuters news agency quoted Interior Minister Arsen Avakov as saying Ukrainian forces had seized control of a television tower.
Further sanctions threatened The West continues to accuse Moscow of provoking the unrest in eastern Ukraine, something Russia denies.
US President Barack Obama, at a meeting with German Chancellor Angela Merkel at the White House, warned of further sanctions against Russia if it did not "change course".
He said the idea that the unrest was caused by a "spontaneous uprising" was "belied" by the militants' use of missiles.
Eastern Ukraine has a large Russian-speaking population. It was a stronghold for President Viktor Yanukovych before he was overthrown by pro-Western protesters in February.
Russia then annexed the Crimean peninsula - part of Ukraine but with a Russian-speaking majority - in a move that provoked international outrage.
The crisis has plunged East-West relations to their lowest point since the Cold War ended in the early 1990s.
http://www.bbc.com/news/world-europe-27265514
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