Ukraine's defence minister has stated that a "great war" has broken out between the country and Russia, one which could claim the lives of "tens of thousands".
"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 on his Facebook page.
Geletey said Kiev was ready to "immediately mount defences against Russia, which is trying not only to secure positions held by terrorists before but to advance on other territories of Ukraine".
His comments come as Ukrainian troops retreated from the strategic Luhansk airport and representatives from Kiev, Moscow and the separatists held negotiations in the Belarussian capital of Minsk.
At the meeting, rebel representatives reportedly requested that Kiev allows the separatist regions of Donetsk and Luhansk a "unique procedure" to integrate more closely with Moscow.
Elsewhere, Russian President Vladimir Putin said European leaders were choosing to ignore Ukraine's "direct targeting" of civilians in the conflict.
Ukrainian President Petro Poroshenko accused Russia of "direct and open aggression" in the eastern rebel-held cities of Luhansk and Donetsk.
In recent weeks, the Ukrainian army had been near to capturing the rebel-held city of Donetsk but a series of military setbacks during the separatist offensive, especially the opening of a new front in the country's south, has forced them back.
Ahead of a Nato summit in Wales, the group's chief, Anders Fogh Rasmussen, said the military alliance must increase its presence in eastern Europe to counter Russian aggression.
"We must face the reality that Russia... considers Nato an adversary," he said. "We cannot afford to be naive."
The massing of thousands of Russian troops on Ukraine's border and reports of Russian incursions into Ukraine has put a spotlight on Moscow's intentions in the post-Soviet states bordering the member nations of the Nato military alliance.
UN agencies estimate more than 2,600 people have been killed in the fighting between pro-Russian separatists and the Ukrainian military since April.