Ukraine's border guard service says hundreds of pro-Russian insurgents have attacked one of its bases in the eastern Luhansk region.
The border service said the battle was still going on hours after the insurgents tried to storm the camp in the early hours of June 2.
It said at least five rebels had been killed and eight injured. It also said seven border officers had been injured, some critically.
A separatist spokesman confirmed only one injured rebel.
The border service accused the insurgents of hiding in surrounding apartment blocks and on roofs and of using civilians as "human shields."
A spokesman said the border guards eventually received air cover from fighter bombers that managed to destroy "two mortar crews of militants."
Fresh fighting was also reported in Slovyansk, a rebel stronghold in neighboring Donetsk region.
The Donetsk and Luhansk regions -- where the separatists have declared their own "independent" republics -- have been the scene of clashes between government forces and pro-Russia separatists for weeks.
The fighting has escalated in the past week, with rebels trying to seize the Donetsk airport and shooting down a military helicopter, killing 14 troops.
Russian Foreign Minister Sergei Lavrov once again demanded an immediate end to the fighting in eastern Ukraine.
He said Russia will submit a resolution in the UN Security Council that demands the start of negotiations and the establishment of a "humanitarian corridor" to allow civilians to leave the area of military activity.
Lavrov, in comments in Moscow on June 2, said that Western countries had assured Moscow that the situation in Ukraine would improve after the country's May 25 presidential election, which saw billionaire Petro Poroshenko win the vote.
But Lavrov says that, instead of improvement, "everything is happening in exactly the opposite way."
Moscow holds the Security Council's rotating chairmanship this month.
Meanwhile, a senior U.S. official visited Kyiv on June 2 and reiterated that Washington and its allies are ready to provide further assistance to Kyiv.
U.S. Assistant Secretary of Defense for International Security Affairs, Derek Chollet, did not elaborate on the nature of the assistance.
With reporting by AP,dpa, and UNIAN