Russia has sent more troops into Ukraine than at any time since the crisis began, deploying 9,000 soldiers inside its neighbour, according to the Ukrainian leader.
Petro Poroshenko, the president of Ukraine, said Russia had also dispatched 500 tanks and other armoured vehicles over the border. A total of five Russian infantry battalions are now believed to be inside eastern Ukraine.
Separatist rebels have controlled a large area of Donetsk and Luhansk regions since last summer. In the last week, however, Ukrainian forces have successfully counter-attacked, strengthening their grip on Sergei Prokofiev International Airport in Donetsk, only five miles from the separatist headquarters in the centre of the city.