Four Ukrainian soldiers have been killed and an army helicopter shot down by pro-Russian militants near the eastern city of Sloviansk, Kiev says.
It 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, a BBC correspondent says.
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.
'Lull in fighting'
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 defence 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.
A BBC team which has reached the centre of Sloviansk says there has been a lull in the fighting between about 800 pro-Russian militants and Ukrainian troops.
On Sunday, Ukrainian Prime Minister 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.
Moscow called on Ukraine and the international community to step up "joint efforts" to end "racism, xenophobia, ethnic intolerance, (and) the glorification of the Nazis" - a reference to extremists Russia claims are committing "mass" rights violations in Ukraine.
Kiev describes the claims as blatant propaganda, and instead accuses the Russian authorities of turning their country into an authoritarian state.
Ukraine's President Viktor Yanukovych was overthrown by pro-Western protesters in February.
Russia then annexed the Crimean peninsula - part of Ukraine but with a Russian-speaking majority - in a move that provoked international outrage.
Despite the latest violence, Ukraine plans to hold a presidential election on 25 May.