Ukraine\’s border guard service says hundreds of pro-Russian insurgents have attacked one of its bases in the eastern Luhansk region.
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.
It 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.
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.
Russian news agencies reported that five people were killed Monday when a warplane fired on the regional administration building inside the city of Luhansk, which currently houses officials of the separatists\’ self-styled Luhansk People\’s Republic.
Against the backdrop of ongoing violence, Russian Foreign Minister Sergey Lavrov said Monday that Russia will submit a draft United Nations Security Council resolution Monday calling for the creation of "humanitarian corridors" in eastern Ukraine, to allow residents to escape the fighting.
Lavrov told reporters the resolution also calls for an end to the ongoing violence in that region, and said the situation there is worsening.
"Our Western colleagues have been telling us for quite a long time that as soon as the presidential election is held in Ukraine the situation will calm down. But everything is happening in exactly the opposite way," Lavrov said.
President-elect Petro Poroshenko won election on May 25.
Also Monday, Russia\’s state-run gas company, Gazprom, delayed a possible shutoff of gas supplies to Ukraine that would impact other parts of Europe as well.
Ukraine paid $786 million to cover some of its gas debt, earning a one-week delay in Russia\’s plan to not ship any more gas unless it is paid for in advance.
The two sides were due to continue negotiations on the issue Monday in Brussels.
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.