At least 38 people are reported to have been killed in a fire in the Ukraine port city of Odessa. Earlier two pilots died when pro-Russian separatists shot down two Ukrainian helicopters in Slaviansk.
The Odessa deaths came as clashes reupted between Ukrainian government supporters and pro-Russians.
Earlier on Friday, amed men in Slaviansk took up positions around the city as residents fortified barricades after Ukrainian troops launched a raid to try to retake the rebel stronghold.
The fighting began at 4am, officials and local residents said. Ukrainian troops could be in seen in APCs in a southern suburb of Slaviansk.
Troops surrounded the eastern city, sending in military helicopters, two of which were shot down by pro-Russian rebels, killing two people.
“As a result of the shooting, two troops from the Ukrainian army were killed,” the ministry said in a statement. A third helicopter, carrying medics, was hit and a medic wounded.
Ten hours later, the city was largely quiet, with shops shut and armed separatists in control of the streets.
Kiev said the firing of missiles that brought down its helicopters was evidence that Russian forces were present in the town. Moscow denies that its troops are on the ground.
Officials said that two helicopters had been shot down by shoulder-launched anti-aircraft missiles while on patrol overnight around Slaviansk.
Michael Clarke, director general at Royal United Services Institute (Rusi), told Channel 4 News that “ground-based fire and/or rocket-propelled grenades were fairly unlikely [to have been used] and they do not have a great record at bringing down military helicopters”.
Mr Clarke said: “[It’s] much more likely [to be] a Russian-supplied, or locally stolen, MANPAD (man-portable air-defence system) and the most likely would be the shoulder-launched SA16 (postable surface-to-air missile) which is ubiquitous and has appeared in many conflicts around the world bringing down helicopters, or possibly an SA18, which is very modern… if it was Russian supplied.
“But a 16 or an 18 is well capable of bringing down an Mi24 Hind. SA16s certainly exist within the Ukrainian stockpile of weapons and would be in existence in eastern Ukraine.”
Russia President Vladimir Putin‘s spokesman on Friday accused Kiev of firing on civilians from the air in a “punitive operation” that destroyed an international peace plan.
Russia was “extremely worried” about the fate of Russians in the city, including an envoy sent to help free German and other foreign hostages, the Kremlin spokesman said.
The dramatic language seems to raise the stakes, as Moscow has tens of thousands of troops massed on the border and claims the right to invade if needed to protect Russian speakers.
Dr Igor Sutyagin, a research fellow at RUSI, said: “Slaviansk is at the centre of an escalating game of deterrence that both Kiev and Moscow are playing against each other.
Mr Sutyagin added: “Kiev knows that it has a strategic reserve of Kalashnikov assault rifles and other light weapons stored in Ukraine as a mobilisation reserve dating back to Soviet times.
“Since the stockpile consists of up to 5 million weapons, the prospect would be a nightmare for Russian military planners if they realistically prepared to move into eastern areas of Ukraine. The stark fact is that at least half the strategic stockpile of light weapons on Ukrainian territory is concentrated near Slaviansk.”
The European Union said it was watching events in eastern Ukraine with growing concern. But Kiev is not a member of Nato and western leaders have made clear they will not fight to defend Ukraine.
Kremlin accounts of grave threats to civilians highlight the risk of a Russian move to seize territory ahead of a vote the rebels aim to hold on 11 May seeking a mandate to break with Kiev, like one held in the Crimea region before Moscow annexed it in March.
For Russians, the Kremlin’s rhetoric of “fascists” in Kiev launching a “punitive operation” evokes the depredations of Nazi German invaders in world war two, being given extensive state media coverage as next week’s anniversary of the Soviet victory is used to foster national pride and nostalgia.
On the square outside city hall in Slaviansk, about 100 people gathered on Friday and said they were appealing to Mr Putin to send troops to help them.
On the town’s southern outskirts, eight Ukrainian armoured personnel carriers cut off the road but faced a cordon two deep of local residents shouting at them to go home. Some rebels threw up new barricades of felled trees.
Mr Putin’s popularity has soared with the seizure of Crimea and talk of restoring Moscow’s former empire. This week he restored the Soviet-era tradition of holding a May Day parade on Red Square, where marchers carried banners hailing the acquisition of Ukrainian territory.