Ukraine's interim PM has accused the security services of failing to stop violence in the southern city of Odessa that left more than 40 people dead.
Arseniy Yatsenyuk, on his way to Odessa, told the BBC there would be a "full, comprehensive and independent investigation" into Friday's events.
Most of the victims were pro-Russian separatists who had barricaded themselves inside a building.
Mr Yatsenyuk has blamed pro-Russian groups for "provoking the unrest".
Dozens of people were arrested after the unrest. On Sunday, hundreds of pro-Russians gathered outside Odessa's main police station demanding their release, and there were reports of scuffles breaking out on the streets.
Some 42 people died in Odessa on Friday, most of them in the fire at the Trade Unions House, where separatist protesters had barricaded themselves in following running street battles with pro-Kiev activists.
Mr Yatsenyuk said the security service and law enforcement office had done "nothing to stop this crackdown", saying they were "inefficient and they violated the law".
The police chief of the Odessa region had been removed, he said, and the prosecutor's office had started an investigation into "every single police officer".
He accused Russia and pro-Russian protesters of orchestrating "real war... to eliminate Ukraine and eliminate Ukrainian independence".
Asked about pro-Russian groups who have taken over many buildings in towns in the east, Mr Yatsenyuk said: "We haven't entirely lost the control... much will depend on the local population, whether they support peace and security."
Mr Yatsenyuk was speaking as Ukrainian troops encircled Sloviansk in the east, where the government is trying to wrest control from the separatists.
The BBC's Sarah Rainsford, in the regional capital, Donetsk, says that while it appears the Ukrainian forces have sealed off the roads in and out of the town, they are moving around it and concentrating on smaller towns nearby.
Our correspondent spoke to a resident of Sloviansk who said people there were expecting it to be stormed.
Gunfire was reported overnight in Kostyantynivka, where one separatist checkpoint was dismantled, and in Mariupol as Ukrainian forces tried to reclaim government offices.
There was heavy fighting in the town of Kramatorsk on Saturday, with the interior ministry saying the army had retaken a television tower.
Kiev officials said at least two people were killed in the town, although Russian state television reported 10 deaths.
Ukraine's National Security and Defence Council chief Andriy Parubiy said on Sunday that the military would expand the "active stage of the operation in towns where extremists and terrorists are carrying out illegal activities".
Ukraine's pro-Moscow President Viktor Yanukovych was overthrown by pro-Western protesters in February.
Russia responded by annexing the Crimean peninsula - part of Ukraine but with a Russian-speaking majority - in a move that provoked international outrage.
Moscow has said it will act to protect Russian speakers wherever they are threatened, but denies it is engineering the unrest.