Pro-Russian militants near the eastern city of Sloviansk have killed four Ukrainian soldiers and shot down an army helicopter, Kiev says.
Kiev says the gunmen used heavy weapons against Ukraine’s units involved in the “anti-terror” operation. Casualties were also reported among the rebels.
The militants are reported to have retreated towards the city centre which is now quiet.
The rebel stronghold remains sealed off by Ukrainian troops.
Pro-Russian militants have seized government buildings in a dozen or more Ukrainian cities in the east.
Kiev accuses Moscow of supporting and arming the gunmen – a claim denied by the Kremlin.
Speaking to reporters near Sloviansk, Ukrainian Interior Minister Arsen Avakov accused the militants of trying to ambush government forces on the outskirts of Sloviansk on Monday morning.
He blamed the rebels for using heavy weapons, including mortars.
The Ukrainian military later said that four soldiers had been killed and about 30 injured.
In a statement, the defense ministry confirmed that a helicopter has been shot down near Sloviansk.
The crew of the aircraft survived, largely because the helicopter crashed into a river, the ministry added. It was the third Ukrainian military helicopter to be downed over the city in recent days.
On Sunday, Ukraine’s PM Arseniy Yatsenyuk accused Russia of executing a plan “to destroy Ukraine and its statehood”.
He was speaking two days after dozens were killed in violence in the southern Ukrainian port city of Odessa.
“Russia’s aim was to repeat in Odessa what is happening in the east of the country,” he said, insisting Kiev had not lost control of the region.
The clashes on Friday left more than 40 dead, mostly pro-Russian separatists killed in a building fire.
Russia warned on Monday that failure to halt the escalating unrest would threaten peace across Europe.
Despite the latest violence, Ukraine plans to hold a presidential election on May 25.
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