Ukrainian President Petro Poroshenko has warned that he may end a ceasefire declared on Friday after pro-Moscow separatists shot down a Ukrainian military helicopter.
Nine servicemen were killed in the attack.
He has instructed government troops to fire back at insurgents “without hesitation”.
Meanwhile, during an official visit to Austria, Russian President Vladimir Putin has commented on his decision to withdraw a decree to authorise the use of military force in Ukraine.
He said he considered the length of the ceasefire to be too short.
“The cancellation of this decision doesn’t mean that we’re not willing to observe the situation in Ukraine very carefully. We’ll definitely protect the ethnic Russians in the country and the Ukrainian people that feel connected to the great Russian people, language and culture,” said Russian President, Vladimir Putin.
Fighting has continued in the Donetsk Region on Tuesday.
At an army checkpoint outside Slovyansk, Ukrainian soldiers said their position has come under repeated sniper and mortar attacks.
Meanwhile, at a pro-Moscow separatist checkpoint outside the city of Kramatorsk rebel gunmen have claimed Ukrainian troops were those responsible for violating the terms of the ceasefire.