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 worst East-West standoff since the Cold War.
The volunteer National Guard said gunmen had sprung a surprise raid 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 and order its troops to "protect" the east's ethnic Russians.
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.
- 'This cannot continue forever' -
Cash-strapped Ukraine has until midnight 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 balked 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 percent 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'
"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 Europe's worst security crisis in decades erupted in March with Russia's seizure of Ukraine's Crimea peninsula.
Putin had rejected the legitimacy of the interim team that rose to power in February and not been reported to have spoken to any Ukrainian official since the Kiev uprising.
The unrest in the heavily Russified industrial regions of Lugansk and Donetsk -- both self-proclaimed independent republics seeking to fall under Kremlin rule -- has claimed around 200 lives and seen abductions by gunmen of Westerners and reporters they suspect of being spies.
Ukrainian officials said they were now working hard to secure the release of four European civilian observers who went missing at a rebel roadblock outside Donetsk on Monday.
The leader of pro-Moscow insurgents in the city of Slavyansk on Thursday confirmed his forces had detained the four observers from the Organization for Security and Cooperation in Europe (OSCE).
The four "are all fine," said Vyacheslav Ponomaryov, the self-proclaimed "people's mayor" of Slavyanksk, in comments to the Interfax news agency.
"We had told them not to travel anywhere for a time but these four turned out to be very keen. Of course they were detained."
"No one arrested them. We detained them. Now we will work out who they are, where they were going and why, and we will let them go," Ponomaryov said.
He suggested that the team could have been involved in espionage.
The OSCE said it had also briefly lost contact on Wednesday with another 11-member team of unarmed monitors who were "stopped at a roadblock" west of Donetsk before being released.
Swiss President and OSCE chief Didier Burkhalter slammed the detention of observers in eastern Ukraine as "acts of sabotage".
In a statement, Burkhalter called for their "immediate and unconditional release."
- Chechen connection -
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.
"Chechens are Russian people," the self-proclaimed "Donetsk People's Republic" government head Aleksander Borodai told reporter.
"They consider this land their own motherland, just like any other city, town or village in the Russian Federation."
Kiev has long argued that the rebel commanders had links to Russia and did not represent the true will of the people of eastern Ukraine.