Police in the Ukrainian city of Odessa said dozens of people have died as the result of a fire in a building that broke out during a clash between pro-Russia demonstrators and supporters of the central government in Kiev.
The fire broke out in a trade union building, but police did not give details on how it started.
Police earlier said at least three people had died in a clash between the two sides.
Until now, Odessa, a key port on Ukraine's southern Black Sea coast, had largely been free of the unrest that has been gripping eastern parts of Ukraine in recent weeks.
Several people were also wounded today when pro-Russian activists attacked supporters of Ukrainian unity marching through Odessa, deepening rifts in the largely Russian-speaking port city.
The opposing sides have clashed before in the Black Sea port, but the battles have never resulted in deaths and some residents said they feared both sides may now try to seek retribution.
The regional police said the three were killed, at least one by a gunshot to the chest, when a march by the pro-Kiev demonstrators was ambushed. Petrol bombs, paving stones and explosive devices were thrown during the clashes, they said.
Dmytro Spivak, a local parliamentarian, told Ukrainian television that four young supporters of the authorities in Kiev had been killed.
“It is abundantly clear that the pro-Russian side was very well armed, well organised and that this action was planned long ago,” he said, adding the police did little stop the clashes. “I will say one thing to [Russian President Vladimir] Putin; forget about Odessa.”
Pro-Russian rebels had earlier shot down two Ukrainian helicopters, killing two crew, as troops tightened their siege of separatist-held Slaviansk in what Moscow called a "criminal" assault by Kiev that wrecked hopes of peace.
President Vladimir Putin’s spokesman said the Kremlin was “extremely worried” that it had not heard from an envoy Mr Putin had sent to the eastern city to help free foreign hostages. He said the “punitive operation” mounted by Ukrainian forces had destroyed a peace plan agreed with western powers two weeks ago.
The Ukrainian Defence Ministry said in a statement that two Mi-24 helicopter gunships were shot down by shoulder-launched anti-aircraft missiles while on patrol overnight around Slaviansk, a city that rebels have turned into a heavily-fortified stronghold. Two airmen were killed and others wounded. The attack helicopter normally has two crew but can carry more.
Other Ukrainian officials and the separatist leader in Slaviansk had said earlier that one airman was taken prisoner.
A third helicopter, an Mi-8 transport aircraft, was also hit and a serviceman wounded, the Defence Ministry said. The SBU security service said this helicopter was carrying medics.
Eight hours after Reuters journalists in Slaviansk heard shooting break out and saw one helicopter opening fire, the city of 130,000 was quiet, with shops shut and armed separatists in control of the streets while Ukrainian forces in armoured vehicles had taken up positions on the outskirts of town.
Ukrainian officials said troops overran rebel checkpoints around the city in an operation launched before dawn this morning and it had been “tightly encircled”. They pointed to the heavy fire that hit the helicopters as proof of the presence of Russian forces, despite repeated denials from Moscow that it has troops on the ground or is controlling the uprising.
Mr Putin’s spokesman heaped blame on the Ukrainian government, which took power two months ago after pro-western protests forced the Kremlin-backed elected president to flee to Russia.
Noting that Mr Putin had warned before that any “punitive operation” would be a “criminal act”, Dmitry Peskov told Russian news agencies that this was what had now happened at Slaviansk, where separatists seeking independence or annexation by Moscow are holding seven foreign European military observers.
Saying Mr Putin had sent an envoy, Vladimir Lukin, to southeast Ukraine to negotiate their release, Mr Peskov said Mr Lukin had not been heard from since the Ukrainian operation began.
“While Russia is making efforts to de-escalate and settle the conflict, the Kiev regime has turned to firing on civilian towns with military aircraft and has begun a punitive operation, effectively destroying the last hope of survival for the Geneva accord,” he said, referring to a deal on April 17th signed by Russia, Ukraine, the United States and the European Union.
Under that agreement, separatists were supposed to lay down their arms and vacate the public buildings they have occupied in about a dozen towns they have seized across the Russian-speaking east. Since then, however, they have tightened their grip.
Gunmen in Slaviansk said they were holding their ground against Ukrainian forces that have struggled to assert control over the past month. Some threw up new barricades of felled trees. And dozens of civilians blocked a road against a unit of Ukrainian armoured vehicles, telling troops to go home.
Reuters journalists in the city heard shooting from shortly after 4 am local time and saw a military helicopter open fire. Towards midday, the city was quiet, shops were shut but rebel gunmen appeared to be still in tight control of the streets. Ukrainian troops were at a halt in the suburbs.
The SBU said the deadly use by the separatists of shoulder-launched anti-aircraft missiles was evidence that “trained, highly qualified foreign military specialists” were operating in the area “and not local civilians, as the Russian government says, armed only with guns taken from hunting stores”.
On his Facebook page, Interior Minister Arsen Avakov posted: "The goal of our anti-terrorist operation and, at the same time, our demands to the terrorists are simple: "Free the hostages, lay down weapons, vacate administrative buildings and get municipal infrastructure back to normal."
He added: “We are ready to negotiate with the protesters and their representatives. But terrorists and armed separatists (will get) only inevitable retribution.”
He urged local people to stay indoors and said Ukrainian forces, from the Interior Ministry, National Guard and the armed forces, had orders not to fire on residential buildings.
Support for the separatist movement is patchier in eastern Ukraine than it was among the majority ethnic Russian population of Crimea, which Moscow seized and annexed in March. However, many are fearful of the new authorities in Kiev and have little faith in Ukraine after 23 years of post-Soviet independence marked by rampant corruption and poor living standards.
“Shells came into my garden,” said one local man, Gennady. “They say that they have come to defend us. But who from?” he said of the Ukrainian forces. “Civilians must stop them.”
The intervention in Ukraine has been enormously popular in Russia. One opinion poll on Wednesday showed 82 percent support for Mr Putin, his highest rating since 2010.