Hundreds of separatists in eastern Ukraine are continuing their assault on a border command centre near the city of Luhansk, the border agency says.
It says five militants were killed and eight wounded when the centre came under sustained attack. Seven border guards are said to have been wounded.
A Ukrainian military aircraft was called in to support the centre.
In Luhansk, there was an explosion in the main regional building seized by the separatists several weeks ago.
t was not immediately known what caused the blast. Unconfirmed reports say there were casualties.
Pro-Russian groups accused Ukraine’s military of carrying out an air strike.
Kiev denied the claim, suggesting that separatists in the building could have mishandled a portable anti-aircraft missile system.
In Moscow, the Russian foreign ministry said in a statement: “The Kiev authorities committed another crime against their own people.”
In other developments:
Ukraine’s electoral commission officially declared Petro Poroshenko the winner of the 25 May presidential elections, saying he secured 54.7% of the vote
Russia’s Gazprom gave Ukraine more time to settle its gas bill after receiving a part payment. The energy giant had previously threatened to halt shipments of gas unless Ukraine paid its bill in full
Moscow reportedly warned it could pull out of the partnership agreement with Nato and take “measures of a military character” should there be major deployments in eastern and central Europe
British PM David Cameron will hold talks with Russian President Vladimir Putin on Friday to urge Moscow to help de-escalate the situation in eastern Ukraine, Mr Cameron’s spokesman said
A spokesman for Ukraine’s border agency, Oleh Slobodyan, said the militants had attacked the border centre with heavy weapons early in the morning.
As many as 500 pro-Russian gunmen are believed to be involved in the assault.
The agency says fighting is continuing and that the attackers – including snipers – are firing from residential buildings, which makes it difficult for Ukrainian troops to respond.
The attackers – reported by the AP news agency to be wearing uniforms – have promised safety for Ukrainian officers if they surrender and put down their arms.
There was a brief ceasefire, but the battle later resumed.
Correspondents say the separatists have become more and more aggressive in their assaults on government-held positions as they try to obtain weapons and ammunition from Ukrainian forces.
For weeks, Russian-speaking eastern Ukraine has been the scene of deadly clashes between government troops and pro-Russian insurgents who have taken over key buildings across the region.
Ukrainian President-elect Petro Poroshenko has said that his priority is to bring fighting in the east of the country to an end.
“We must make every effort to make sure that no more Ukrainians die at the hands of terrorists and bandits,” he said on Friday.
The conflict has intensified in recent days. The rebels say they lost up to 100 fighters when they unsuccessfully tried to seize Donetsk’s international airport on 26 May.
Pro-Russian separatists in the regions of Donetsk and Luhansk declared independence after referendums on 11 May, which were not recognised by Kiev or its Western allies.
The rebels took their cue from a disputed referendum in Crimea, which led to Russia’s annexation of Ukraine’s southern peninsula.