The volunteer National Guard said gunmen had sprung a surprise raid yesterday on their barracks in the flashpoint Lugansk region near Russia that had been under effective rebel control since early April.
"There have been losses both in the ranks of the military unit and the attacking side," the National Guard said in a brief statement.
The militants' strike came the day after Ukraine claimed to have regained control of the rust belt region's main international airport in Donetsk after its most ferocious attack of the seven-week campaign which left more than 40 guerrillas dead.
The Kremlin reaffirmed yesterday that it "respected" the will of Ukraine's voters but also denounced the army's "provocative" actions as another step towards strife and discontent.
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"There can be no justification for the punitive operation being conducted by the Kiev authorities," Russian Foreign Minister Sergei Lavrov said.
Cash-strapped Ukraine has until midnight today to pay Russia USD 2 billion under an EU-brokered agreement or face a halt in gas supplies next week that would also impact parts of Europe.
"We will hold talks with Putin in order to ease the situation and make peace. When and where these talks will take place is not yet decided," he told Germany's Bild daily.
The political veteran was responding to a question on whether talks with Putin were planned for next week's D-Day commemorations in France to which both he and the Russian leader are invited.
Rebel commanders meanwhile admitted that fighters from Chechnya - a Muslim region of Russia that once fought for its independence but is now under a Kremlin-backed strongman's control - were enlisted in the separatist brigades.