The Russian-backed leader of the breakaway Donetsk region said on Wednesday he wanted to peacefully settle its borders with Ukraine but reserved the right to ask Russia for help.
Denis Pushilin, who heads the self-proclaimed Donetsk People's Republic that was recognized by Russia this week, said he favored dialog with Ukraine in the first instance.
But he told a news conference the situation in their long-running conflict had become critical, and the separatists would win with support from "big Russia."
Russia's recognition of Donetsk and the neighboring self-proclaimed Lugansk People's Republic on Monday was declared illegal by Western countries, which have imposed new sanctions against Moscow.
"Aggression from the Ukrainian side is growing," Pushilin said, accusing Kyiv of staging "provocations" and causing civilian casualties.
Ukraine denies accusations by the separatists that it is stoking the conflict with the aim of recapturing their territory by force.
Pushilin said an evacuation of women and children which began last week was necessary to enable the separatist army to focus on defending the borders of their territory.
"Mobilization is growing, we see that. Both former militiamen and ordinary men who cannot stand aside when the fate of Donbass is being decided," he said.
Speaking alongside Pushilin, Andrey Turchak, a senior member of Russia's ruling party, said no force in the world could change the legal result of Russia's recognition.
Turchak said it was unlikely that any agreement could be reached with the current Ukrainian government. He told Pushilin that Russia would not abandon its people.