Ukrainian Defense Minister Valeriy Geletey accused Moscow of launching a "great war" that could claim tens of "thousands of lives."

Ahead of talks with the separatists later this week, Geletey said Russia was behind a counter offensive by pro-Russia militants in the eastern Donetsk and Luhansk regions.

Geletey, in a message posted on his Facebook page, vowed that his country would: "immediately mount defenses against Russia, which is trying not only to secure positions held by terrorists before but to advance on other territories of Ukraine."

"A great war arrived at our doorstep, the likes of which Europe has not seen since World War II," he said.

Among the rebel advances has been the seizure from government forces of Luhansk airport.

Geletey's warning came as preliminary talks involving separatist leaders began on Monday. Further discussions are set to take place later in the week.

The European-backed talks in Minsk are only at a preliminary stage

A Russian news agency quoted representatives of the separatists at the talks as demanding that Kyiv provide it with a special status allowing them closer integration with Russia.

'Statehood' of eastern regions

That followed Russian President Vladimir Putin on Sunday saying that the issue of "statehood" should be brought up in the talks. Putin also accused the EU, which is considering new sanctions against Moscow, of ignoring the "direct targeting" of civilians by Ukrainian forces in the conflict, which has killed more than 2,600 people since mid-April.

NATO chief Anders Fogh Rasmussen said on Monday that the alliance would consider the creation of a high-readiness force and the stockpiling of military equipment in Eastern Europe to help protect member states. According to NATO, Russia had more than 1,000 of its troops deployed and a further 20,000 massed along the border. While Ukraine is not a NATO member, other former Soviet republics, such as Estonia, Latvia and Lithuania, are.

"The Readiness Action Plan will ensure that we have the right forces and the right equipment in the right place, at the right time," Anders Fogh Rasmussen said on Monday . "Not because NATO wants to attack anyone. But because the dangers and the threats are more present and more visible. And we will do what it takes to defend our allies."

President Barack Obama and leaders of NATO's other member nations open a summit meeting on Thursday in Wales to seek a response.

rc/jm (AFP, AP, Reuters)