Ukrainian forces have suffered fresh losses in a rebel raid in the separatist east as the newly-elected president promised to hold his first talks with Russia's Vladimir Putin on easing the crisis.
The volunteer National Guard said gunmen had sprung a surprise raid on Wednesday 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.
Some analysts think Kiev has ratcheted up its offensive believing that the 40-point margin of Petro Poroshenko's victory at the polls on Sunday will make it hard for Moscow to question his legitimacy.
The Kremlin reaffirmed on Wednesday that it "respected" the will of Ukraine's voters but also denounced the army's "provocative" actions as another step towards strife and discontent.
"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 on Thursday to pay Russia $2 billion under an EU-brokered agreement or face a halt in gas supplies next week that would also impact parts of Europe.
Ukrainian Prime Minister Arseniy Yatsenyuk spent Wednesday in Berlin conducting urgent energy security talks with German Chancellor Angela Merkel, less than a week before Europe faces another possible reduction in Russian gas supplies.
Yatsenyuk said in a statement read alongside Merkel and the leaders of Georgia and Moldova that "our goal is that Ukraine becomes a European country" and that "a new Berlin Wall" never goes up again.
Russia and Ukraine launched their third gas war in less than a decade after the ouster of the Kremlin-backed president in February prompted Moscow to nearly double Kiev's gas price.
Ukraine refused to pay in protest and has since baulked at the terms of an interim deal negotiated with the help of a top EU energy official that would have seen Russia receive a down payment on its debt by Thursday.
Putin pointed out on Wednesday, as the deadline loomed, that Russia had not received any payments since November but was still willing to negotiate a lower price for Ukraine - if it was paid on time.
"This cannot continue forever," Putin told a government meeting. "Everyone understands that perfectly well."
About 15 per cent of all gas consumed in Europe is pumped in from Russia through the ex-Soviet state.
Poroshenko - a 48-year old confectionery tycoon who backs closer ties with Europe but once held a top cabinet post under ousted pro-Russian president Viktor Yanukovych - said he would conduct direct negotiations with Putin soon.
"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.
The June 6 Normandy gathering will also be attended by US President Barack Obama, providing Western leaders with their first chance to speak to Putin since the crisis began.
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