East Ukraine is braced for more unrest, as troops from Kiev prepare to continue "anti-terror" operations and pro-Russian gunmen reinforce barricades.
Ukrainian soldiers advanced on several cities on Saturday to regain control of buildings seized by the gunmen.
But in at least one city, Luhansk, the rebels made further gains, storming a security services base.
Clashes in the southern city of Odessa in which at least 42 people died in a fire have further raised tensions.
Pro-Russian protesters had gathered at Odessa's Trade Unions House after running battles with pro-Kiev activists.
The building caught fire, killing many of the protesters.
A Russian government spokesman accused Ukraine's government of encouraging nationalist extremists and said it would be "absurd" for the country to hold a planned presidential election on 25 May.
The violence in Odessa was the most serious in Ukraine since February when more than 80 people were killed during protests in Kiev against the ousted President Viktor Yanukovych.
The blackened trade union building was ringed by police on Saturday and there were some scuffles as groups chanting pro-Russian slogans clashed with government supporters.
'A FRATRICIDAL CONFLICT'
US Secretary of State John Kerry and his Russian counterpart, Sergei Lavrov, again spoke by phone about the crisis.
Lavrov urged Kerry to put pressure on Kiev to stop its military operation, which he said risked "plunging the country into a fratricidal conflict".
Kerry said Moscow should stop backing the pro-Russian separatists and warned that Russia risked incurring further sanctions from the West.
Both men also discussed the possibility of greater involvement by the Organisation for Security and Co-operation in Europe (OSCE) in trying to find a solution to the crisis.
Meanwhile, fighting intensified in eastern Ukraine on Saturday.
There were more clashes between Ukrainian forces and pro-Russian militants in the town of Kramatorsk, where at least two people were said to have been killed.
The Ukrainian interior minister said troops had recaptured a security services building.
Gunfire was also reported overnight in Kostyantynivka and Mariupol as Ukrainian forces tried to reclaim government buildings.
Pro-Russian gunmen in Mariupol set fire to barricades and videos showed a branch of PrivatBank, a bank owned the pro-Kiev governor of Dnipropetrovsk, being burned down.
Fighting also continued on the outskirts of Sloviansk, the stronghold of pro-Russian groups, with reports saying several people had died in clashes.
Despite the violence, seven international monitors held by pro-Russian gunman were released after being held in Sloviansk for a week.
Five Ukrainian officers captured with the observers, who are linked to the OSCE, were also released.
Pro-Russian separatists in Sloviansk said they released the observers "without conditions".
"As I promised them, we celebrated my birthday yesterday and they left. As I said, they were my guests," the self-proclaimed mayor of Sloviansk, Vyacheslav Ponomaryov, said.
One of the observers, German Col Axel Schneider, said the team had been treated "as good as possible" in what was a "miserable situation".