Hundreds of armed rebels attacked a border guards' camp in eastern Ukraine and at least five of the attackers were killed in the fighting, a spokesman for the guards said.
The spokesman, Serhiy Astakhov, told the AP news agency by telephone that a preliminary assessment indicated that eight rebels were also injured during the attack on Monday in Luhansk, a major city not far from the Russian border.
He said seven border guards were also injured, three of them seriously.
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Another border post spokesman, Oleh Slobodin, told the Reuters news agency that the fighting lasted seven hours.
"We have eight or nine wounded. The attackers have five dead and eight wounded," he said.
Rebels massed near the base, who were wearing uniforms, had earlier promised border officers that they would be unharmed if they surrendered and laid down their arms, AP reported.
The initial attack was launched by about 100 rebels but the number of attackers swelled to around 400 a few hours later.
The rebels, who have seized government and police buildings across eastern Ukraine and want unity with Russia, have waged increasingly aggressive attacks on government-held checkpoints and garrisons in an attempt to take weapons and ammunition from Ukrainian forces.
Months of protests last year forced Ukraine's pro-Russian former president Viktor Yanukovich to flee the country.
Many in Ukraine's east are suspicious of the new pro-Western government in Kiev, and protests in favour of greater independence from the capital soon turned into a separatist movement as the Luhansk and Donetsk regions declared independence following hastily called referendum.
The conflict between Ukrainian government forces and rebels has escalated in the past week, with rebels attempting to seize a major airport and shooting-down a Ukrainian military helicopter .