Kiev (AFP) - Ukraine's defence minister warned on Monday that a "great war" had broken out with Russia over his country's future that could claim tens of thousands of lives.

"A great war has arrived at our doorstep -- the likes of which Europe has not seen since World War II. Unfortunately, the losses in such a war will be measured not in the hundreds but thousands and tens of thousands," Valeriy Geletey wrote in a Facebook post.

Russia on Monday again denied either sending or planning to send troops into eastern Ukraine to help separatist rebels pursue their recent counteroffensive against the pro-Western government's forces.

But insurgency leaders have admitted that some off-duty Russian soldiers had already joined their ranks.

NATO has also accused the Kremlin of advancing more than a 1,000 soldiers and heavy weapons across the Ukrainian border in recent days.

Geletey wrote on Monday that "hundreds of Russian soldiers and officers have permanently entered Ukraine's (eastern) 'black earth' region."

But he stressed that "Ukraine has no plans to surrender" and compared the conflict to the "Great Patriotic War" -- the name former Soviet nations use for their fight against Nazi Germany in World War II.