Ukraine's embattled new leader agreed on Friday to immediate crisis talks with rebel commanders and Russia aimed at stemming bloodshed that has threatened his strategic country's survival and ruptured East-West ties.

Clashes in the economically-vital border regions of Lugansk and Donetsk have picked up with renewed vigour since Ukrainian President Petro Poroshenko tore up a 10-day ceasefire earlier this week.

His decision was immediately followed by the launch of a "massive" offensive by Kiev that led President Vladimir Putin to warn that Russia has the right to protect its compatriots in Ukraine.

The head of Ukraine's national security and defence council said government forces had regained control over 23 of the conflict zone's 36 local regions.

Interior Minister Arsen Avakov said troops on Friday had seized back the village of Mykolaivka -- scene of constant clashes that lies just east of the separatist stronghold Slavyansk -- and captured 50 gunmen.

Ukraine's security service also accused Russia of supplying the insurgents with up to 20 additional armoured vehicles in the past four days. Moscow has rejected similar charges levelled earlier by the United States.

But the self-proclaimed "Donetsk People's Republic's" defence minister appeared to confirm facing an onslaught from resurgent government forces of a scale not witnessed since the first battles broke out in April.

Igor Strelkov told the pro-Kremlin LifeNews channel that his units "will be destroyed... within a week, two weeks at the latest" unless Russia helped secure an immediate truce or moved in its troops.

Yet the insurgents showed few signs of abandoning their separatist drive. The military reported the loss of nine soldiers in the latest exchanges of mortar fire -- the heaviest toll since fighting resumed on Monday.

- Unified Western push -

The uprising was sparked by the February ouster of a pro-Kremlin administration and fuelled by Russia's subsequent seizure of Crimea. Relentless shelling and sniper fire have since killed more than 470 people and left Western leaders frustrated by repeated mediation failures.

But the ongoing low-scale warfare on the EU's eastern frontier has also unified the West in its biggest pushback yet against Putin's seeming attempt to reassert command over former Soviet lands.

Russia now faces the threat of devastating economic sanctions should Putin fail to explicitly order the militias to lay down their arms.

Poroshenko's decision to propose the next discussions for Saturday followed yet another telephone exchange with German Chancellor Angela Merkel and French President Francois Hollande.

The Ukrainian leader then told EU foreign policy chief Catherine Ashton that he had "proposed a place and time for the meeting and is waiting the other party's confirmation."

- Donetsk or Minsk? -

Kiev has balked at the idea of holding round-table talks in Donetsk -- a location in which Moscow carries widespread influence and prefers. But the insurgents fear travelling to Kiev or EU member countries for fear of their immediate arrest.

A source at the Organisation and Security and Cooperation in Europe (OSCE) -- a Vienna-based body first formed to preserve peace in the Cold War era and now leading the nascent peace talks -- said Donetsk remained the most likely venue for the so-called Contact Group discussion.

But the number two man in the separatist Donetsk cabinet told Interfax he had been told that Kiev had rejected the city for security reasons and was instead proposing the Belarus capital Minsk.

- Russia as 'junior partner' -

The United States has preferred not to play a direct role in the negotiations this time around after its decision to send senior diplomats during the winter protests in Kiev enraged the Kremlin.

Obama has had regular phone calls with European heads of state and Putin himself that underscored his concern about Russia's expansionist threat, but Washington's long-distance approach has done little to appease Moscow.

"The United States believes that it won the Cold War and that Russia -- a successor to the Soviet Union -- lost," Deputy Foreign Minister Sergey Ryabkov told Moscow's Kommersant daily.

"And from this it concludes that Moscow must obey and conduct itself in international affairs... as a junior partner."