The four-month conflict in Ukraine has reached a critical new phase.
Kiev and western governments are watching nervously to see if Russia will intervene in support of rebel forces, an intention that Moscow denies.
The prime minister of the self-proclaimed Dontesk People's Republic said on Friday that separatists were in the process of receiving almost 150 armored vehicles, and 1,200 trained Russian fighters.
The rebels have been promising a counter-offensive but have yet to launch one.
Foreign ministers of Ukraine, Russia, Germany and France are scheduled to meet Sunday in Berlin, in a possible first step towards a peace agreement.