Ukrainian army has surrounded the largest, population of 1 million, rebel held city of Donetsk but because of chances of enormous civilian loses has halted the attack. Rebels in the city are also well armed with latest Russian arms, even better then Ukrainian army. All of that means that whatever happens and whoever wins it may turn into a huge disaster for the new Ukrainian government.
Also, Russia has massed 20,000 well armed and trained troops on the border. Those troops can reach the city in less then an hour and reinforce the rebels.
Ukrainian army closes in on Donetsk as rebel fighters call on Russia for help