At least 30 people were killed in southern Ukraine on Friday when clashes between pro-Kiev and pro-Russia protesters spread from the east of the country following a government offensive against separatist rebels.
Ukrainian police said 31 people died in the Russian-speaking Black Sea port of Odessa after a trade union building was set ablaze in a violent confrontation between rival groups in the city. Together with several deaths in the course of a government military operation against pro-Russian secessionists in the eastern town of Slavyansk, the events in Odessa made for the worst day of unrest in Ukraine since 100 protesters were killed in demonstrations that led to the ousting of Viktor Yanukovich from the Ukrainian presidency in February.
Many of those killed in Odessa were reportedly pro-Russian demonstrators who stormed the trade union building before a fire broke out. Most died from smoke inhalation, but others leapt out of the windows, some to their deaths.
The deaths are bound to inflame opinion in Moscow, which rounded on Kiev earlier on Friday for launching a "punitive operation" in Slavyansk against its own citizens, an operation that could provide Russia with a reason to launch a military intervention.
Speaking on Ukrainian television late on Friday, Serhiy Pashynsky, head of Ukraine's presidential administration, said the violence in Odessa was a "provocation" of Russia's "FSB" state security agency intended to "destabilise the situation" and "implode our country from the inside".
In a statement, Russia's foreign ministry called the deaths "another manifestation of the criminal irresponsibility of the Kiev authorities". Moscow accused them of "indulging radical nationalists, including Right Sector" who in turn were "organising physical terror campaigns against supporters of federalisation".
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Arseniy Yatseniuk, Ukraine's interim prime minister, told the FT on Thursday of the risks of provoking a Russian military response.
"If you start this kind of very tough operation, you will definitely have civilian casualties. And this is the perfect excuse for President [Vladimir] Putin to say look, these ultranationalists kill Russian-speaking people."
Dmitry Medvedev, Russia's prime minister, wrote on his Facebook page: "People are dying. Blood has been shed. The responsibility for this war against their own people lies with those making the criminal decisions in Kiev. Those in power in the Ukrainian capital should come around and stop killing their own citizens. Otherwise the country's fate may be extremely sad."
At least three people were killed in earlier clashes in Odessa and six in Slavyansk, where Kiev sought to reassert its authority in the east by launching an offensive against armed pro-Russian separatists.