US President Barack Obama has accused Russia of being responsible for the violence in eastern Ukraine, reports GHN based on BBC.
He said the fighting was not the result of a home-grown uprising but of "deep Russian involvement", and new satellite images made its role clear.
Russia denies Nato claims that more than 1,000 Russian troops are fighting with pro-Russia separatists. It accuses Ukraine of attacking its own people.
More than 2,000 people have been killed in four months of fighting.
Nato is to hold an emergency meeting on Friday to discuss the crisis.
It follows Thursday's emergency meeting of the United Nations Security Council in New York.
Pro-Russian separatists have recently opened a new front in the conflict, seizing the south-eastern coastal town of Novoazovsk.
It has raised fears that the Kremlin might want to create a land corridor between Russia and Crimea - a territory annexed by Russia from Ukraine in March.
The rebel advances have left a number of pockets of Ukrainian military surrounded, but overnight Russian President Vladimir Putin called on the rebels to open a "humanitarian corridor" to allow the Ukrainian troops out of the encirclement in order to avoid unnecessary casualties.
Rebel leader Alexander Zakharchenko later told Russian TV that they had agreed to the request.