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.
The operation in Slavyansk, which involved Ukraine's army, interior ministry forces and newly formed national guard, began at about 4:30am local time when automatic gunfire was heard and alarms sounded.
Gun battles erupted when government forces moved to retake checkpoints armed by separatist rebels around the city. Photographs posted on websites showed gunmen in entrenched positions in the centre of town, in preparation for a possible advance by Ukrainian forces.
By early evening Kiev, said it controlled all the main checkpoints around Slavyansk but the centre of the city remained in the hands of pro-Russia forces, the Associated Press reported.
"The operation is moving not as fast as wanted and is complicated because the base for these terrorists are in highly populated cities; they are hiding behind the backs of civilians, hostages and shooting from residential buildings," said Oleksandr Turchynov, Ukraine's acting president.
Vyacheslav Ponomarev, the self-declared "people's mayor" of Slavyansk, posted a video dressed in camouflage and armed with an assault rifle, urging pensioners, women and children to remain indoors, while calling on all men to help defend the city.
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Ukraine's defence ministry said two of its servicemen were killed and several injured when two Mi-24 assault helicopters were shot down in the Slavyansk region by anti-aircraft rockets. A Mi-8 transportation helicopter was also hit by gunfire, the ministry added.
On its website, the ministry posted photographs of four alleged pro-Russia militants lying face down on the ground with their mobile phones and cigarettes from Russia scattered nearby.
The ministry said the men "might be responsible" for the shooting down of the two helicopters early on Friday.
Mr Ponomarev, speaking on video, took credit for shooting down the helicopters. He said three pro-Russia militants from his side were killed in the clashes.
By mid-afternoon, Ukraine's interior ministry set out the demands of what it called an "antiterrorist operation".
"Separatists must immediately put down their arms, free all hostages and seize administrative buildings and end the chaos and pogroms," it said in a statement. "Ukraine's leadership is ready to grant amnesty to all . . . that did not commit capital crimes."
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On April 24, Ukrainian army and interior ministry troops launched an offensive against Slavyansk that killed at least seven people and injured several others, but failed to regain control of the centre of the city or strategic rebel-held checkpoints.
Separatist rebels a week ago seized a bus near Slavyansk carrying members of a German-led OSCE military observer mission, taking seven of its west European members and five Ukrainian colleagues hostage. Mr Ponomarev later described the group as "prisoners of war".
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It emerged on Friday that Mr Putin had sent Vladimir Lukin, an envoy, to Slavyansk to try to negotiate the observers' release.
Russian state TV juxtaposed images from Slavyansk of peaceful pensioners, women and children gathered in the centre of city, with those of Ukrainian forces they accused of being far-right extremists and foreign mercenaries. The state channels said half-a-dozen locals had died in the attack.