Escalated violence in the last two days in the eastern region has pushed Ukraine closer to civil war with the government in Kiev admitting that it is "at war" with the dissenters who have the backing of Russia.
Bloodshed in Odessa, where more than 40 people have died, and at least 10 more deaths in rebel-held towns that came under the Kiev military's counter offensive, have pushed the government on the defensive.
Meanwhile, Russia has warned that it is weighing options after getting thousands of pleas for help from pro-Russian protesters.
With Kiev struggling to gain control over Donetsk, Slovyansk, Luhansk and other cities, and as thousands of protesters in Odessa flooded the streets chanting pro-Russia slogans, the authorities have said the challenge was tougher than previously thought.
"What we are facing in the Donetsk region and in the eastern regions is not just some kind of short-lived uprising – it is in fact a war," said Vasyl Krutov, the head of Kiev's "anti-terrorist" operations in the east.
Kiev's counter offensive in many of the eastern cities failed to halt the advance of pro-Russian protesters. In Donetsk they took control of more government buildings and in Luhansk, rebels declared war on Kiev.
In Odessa, where more than 30 pro-Russian protesters were charred to death in a building blaze as both sides clashed, thousands marched pledging allegiance to Russia.
Responding to the worsening crisis, Russia said it was considering how to respond to calls for help from Russian-speaking people in eastern Ukraine.
"People are calling in despair, asking for help. The overwhelming majority demand Russian help," Kremlin spokesman Dmitri Peskov said.
The Kremlin, which has sent thousands of troops to the border, earlier said it would intervene militarily if the Russian-speaking people of eastern Ukraine were under threat.
The crisis in Ukraine worsened following the ouster of pro-Russian President Viktor Yanukovych and the Russian annexation of Crimea that followed it.