Pro-Russian separatists continued to defy demands for them to lay down their arms as Ukraine’s acting president appealed for the United Nations to send in peacekeeping troops.
Amid growing tensions across the east of Ukraine, protesters appeared to ignore a deadline on Monday morning to leave government buildings or be faced with reprisals by Ukrainian forces.
Protesters are now thought to have occupied state buildings in 10 cities across the Donetsk and Luhansk regions.
In Horlivka a 100-strong crowd attacked the police headquarters in a fresh assault, a witness told Reuters. Video footage on Ukrainian television showed an ambulance treating people who were apparently injured during the attack in the city in the Donetsk region, which has a population of around 300,000 people.
Armed men in masks also took over a military airport outside the flashpoint city of Slovyansk, also in Donetsk.
Later, protesters claiming to be part of a new "Donetsk People's Republic" asked Vladimir Putin to provide assistance as they continue to disobey orders from Kiev.
Speaking at a news conference at the headquarters of the city administration of Slovyansk, a separatist leader called on the Russian President “to personally direct your attention to the unfolding situation and help us as much as you can”, the Reuters news agency reported.
The acting Ukraine president, Oleksander Turchynov, sought to quell the uprising on Sunday night with the threat of a full-scale “anti-terror operation”. In a statement, he promised an amnesty to pro-Russian fighters who surrendered their weapons and left government buildings by 9am (6am GMT) on Monday.
It has also emerged that Mr Turchynov has spoken with UN Secretary-General Ban Ki Moon to suggest that Ukrainian security forces and UN peacekeepers unite in an “anti-terrorist” operation.
However, such a move is unlikely to happen as it would have to be authorised by the UN Security Council, in which Russia holds a veto.
Mr Turchynov has accused Russia of persistent provocation following the departure of Moscow-backed former president Viktor Yanukovich amid months of pro-Western protests.
“The blood of Ukrainian heroes has been shed in a war that the Russian Federation is waging against Ukraine,” he said in a televised address on Sunday. “The aggressor has not stopped and is continuing to sow disorder in the east of the country.”
However, Mr Turchynov has said the Kiev government is “not against” a nationwide referendum to decide how Ukraine should move forward. He said he thought such a poll would confirm people's desire for a united and independent Ukraine.
Pro-separatist rebels have demanded a referendum, but they only want it held in their home region rather than across the country
In other developments, the Russian Foreign Ministry Sergei Lavrov said that Ukraine's Russian-speaking eastern regions should be involved in drafting a constitution that should be put to the referendum.
Mr Lavrov said it was not in Russia's interests for Ukraine to break up, but that Moscow wanted all citizens of the country to be given equal treatment by Kiev.
He denied Ukrainian and US allegations that Russia had undercover agents fomenting unrest in eastern Ukraine, and said he was seeking explanation of media reports that the director of the CIA, John Brennan, had visited Kiev.
Mr Turchynov and other leaders blame Russia, which has already annexed the Crimea region, for inspiring and organising the series of rebellions.
The crisis has brought relations between Russia and the West to their lowest point since the end of the Cold War in 1991 but, with tensions building in the region, there are fears that any full-scale military operation by Ukraine could precipitate an invasion by Russian forces.
Last week Nato released satellite images showing what it said were some 40,000 Russian soldiers near the Ukrainian border, along with long lines of tanks, armoured vehicles, artillery and aircraft ready for action.
Moscow dismissed the pictures, saying they were from last August, but Nato said the images indicated that Russia was ready for action at the drop of a hat.
Meanwhile, British Foreign Secretary William Hague has accused Russia of deliberately destabilising eastern Ukraine.
Speaking on Monday at a meeting of EU foreign ministers in Luxemburg to discuss the crisis, Mr Hague dismissed denials of Russian involvement.
“There can't really be any real doubt that this is something that has been planned and brought about by Russia,” he said. “I don't think denials of Russian involvement have a shred of credibility.”
He added that recent events mirrored Russia’s actions in seizing Crimea from Ukraine: “It has all the appearance of a further gross deliberate, premeditated violation of the independence and sovereignty of Ukraine.”
Following a phone call, Prime Minster David Cameron and German Chancellor Angela Merkel called on Russia to censure the rebels.
“He and Chancellor Merkel agreed about the importance of condemning the illegal occupation of government buildings in eastern Ukraine,” Cameron's official spokesman told reporters.
“They believe the Russian government should be unequivocally condemning that action too."
The UK said it would be pushing for the EU to impose further sanctions against Russia for its actions. A so-called “third phase” of EU sanctions is expected to include restrictions on trade and finance with Russia, potentially with wide-reaching repercussions for the Russian and EU economies.
But Russian Ambassador Vitaly Churkin denied Western and Ukrainian claims that Moscow was behind the violence, and told UN diplomats that Ukraine has been using radical neo-Nazi forces to destabilize its eastern region.
“It is the West that will determine the opportunity to avoid civil war in Ukraine. Some people, including in this chamber, do not want to see the real reasons for what is happening in Ukraine and are constantly seeing the hand of Moscow in what is going on,” Churkin said.
“Enough. That is enough.”
He said after the meeting that he hoped Western powers would pressure Ukraine to rethink its deadline for sending in troops.
“Whether they are going to put an end to this provocation by Kiev, this is their responsibility to prevent further escalation of this crisis,” Churkin said.
Additional reporting by agencies