A Ukrainian Interior Ministry official said at least 30 servicemen were killed Friday after pro-Russia rebels fired on them with missiles.
Zoryan Shkiryak, an adviser to the interior minister, said authorities would react swiftly to punish the perpetrators.
"They will be destroyed or captured and be made answerable to Ukrainian law," Shkiryak said.
Defense Ministry spokesman Vladislav Seleznyov said the attack was on a forward base in a village near the Russian border and confirmed there had been heavy casualties, but did not give any specific figure.
The base was located near one of three border crossings whose closure Russian officials had announced Friday.
Ukrainian government troops have been fighting for more than three months against separatists in eastern Ukraine. In the last two weeks, however, they have cut the amount of territory held by the rebels in half and forced them out of their stronghold in the city of Slovyansk. The rebels have since regrouped in the eastern city of Donetsk and Ukraine has vowed to cordon the area.
Border crossings are of particular concern to both sides. Ukraine says that Russia is supporting and arming the rebels, but Russia denies the charges. The rebels have captured a few of the crossings and Ukraine has demanded them back.
Russian news agencies quoted Vasily Malayev, spokesman for the Federal Security Service in the Rostov region, as saying the border crossings east of Donetsk were temporarily closed late Thursday because of fighting.
Ukraine on Friday said it had regained control of one of those rebel-held crossings.