Ukrainian forces have suffered a series of setbacks as pro-Russian rebels advance in the east of the country.
Officials said nearly 700 soldiers had been taken prisoner since the rebel offensive began.
Earlier, the military said government forces had been forced to withdraw from Luhansk airport, saying they had been attacked by a column of Russian tanks.
Ukraine Defence Minister Valery Heletey said it was now fighting a "great war" with Russia in which tens of thousands could die, the BBC reported.
Mr Heletey said on his Facebook page that the rebels had been defeated and Russia had been forced to begin a full-scale invasion of the region with regular forces.
"A great war has arrived at our doorstep - the likes of which Europe has not seen since World War Two," he said.
"Unfortunately, the losses in such a war will be measured not in the hundreds but thousands and tens of thousands."
Russia has repeatedly denied Ukrainian and Western accusations that it is providing troops and equipment to the rebels.
Meanwhile, Ukrainian and Russian officials are holding talks with separatist rebels and international monitors in Minsk.
Some 2600 people have died in eastern Ukraine since fighting began in April.
The conflict broke out after Russia's annexation of Ukraine's southern Crimea peninsula in March.