At a military academy in Kiev, Ukrainian President Petro Poroshenko says Russia's direct involvement in Kiev's war against the separatists had tipped the balance on the battlefield.
"Direct and open aggression has been launched against Ukraine from a neighbouring state. This has changed the situation in the zone of conflict in a radical way and raises new and more difficult questions for our security bodies," President Poroshenko said.
In turn, Russian President Vladimir Putin is pointing the finger at Kiev's leadership for refusing to enter into direct political talks with the separatists.
Putin also says he hopes "common sense" would prevail in the West over the possibility of imposing additional economic sanctions - that is despite Moscow's denials that it is helping the rebels.
In Berlin, Chancellor Angela Merkel offers no such assurances.
"We cannot accept Russia's behaviour and that's why I believe it is necessary to prepare such sanctions. The disadvantages which could also arise for us are in no way as serious as the disadvantage not to do anything about it," according to Merkel.
Until last week, Ukraine appeared close to crushing the four-month rebellion in the east, but then the rebels opened a new front to the south, and with it, new uncertainty about Ukraine's future.