Pro-Russian activists have attacked another official building in east Ukraine, ignoring a deadline to leave or face eviction by Ukrainian forces.
A crowd stormed a police station in the town of Horlivka, near Donetsk, taking control of the building.
Ukraine's interim president hit out at "aggression" from Russia, but signalled support for a national referendum.
Olexander Turchynov said Kiev was "not against" a vote on the future of the country, a key demand from protesters.
Mr Turchynov also said Ukraine was preparing an "anti-terrorist operation" against gunmen occupying government buildings in Sloviansk and a number of other towns and cities.
His office said that he had suggested the UN could assist in any such operation. This would be highly unlikely as Russia has a veto on the Security Council, which would have to authorise any such action.
Correspondents say people in eastern Ukraine are anxiously waiting to see if Mr Turchynov carries through on his threat to use the army against the pro-Russian groups.
In other developments:
- Moscow denied allegations that Russian agents had been fomenting unrest in east Ukraine
- Vitaly Tsyhanok, the head of Ukraine's anti-terror operations, was sacked after criticism of Kiev's response to the crisis in the east
- A Ukrainian official said the government had not declared a state of emergency over the crisis in the east because it would require suspending the run-up to presidential elections on 25 May
- Britain called on the European Union to agree "further sanctions" against Russia in response to the escalating crisis
'A united Ukraine'
In a televised address to parliament, interim President Turchynov suggested Kiev would be open to moving from a republic into a federation and giving broader rights to Ukraine's Russian speakers.
The pro-Russian groups who have seized government buildings in eastern regions are demanding local referendums on either increased local rights or an option to join the Russian Federation.
But Mr Turchynov stopped well short of giving in to these demands by showing support for a national referendum, of which the outcome is uncertain because most people in Kiev and the Ukrainian-speaking west reject the idea of federalisation.
"We are not against holding a national referendum," Mr Turchynov said. "I am certain that a majority of Ukrainians will support an indivisible, independent, democratic and united Ukraine."
He also used the speech to accuse Moscow of open aggression in the east of the country.
"It's not a confrontation between Ukrainians, but covert and now no longer covert aggression by the Russian Federation against our country," he added.
Russian Foreign Ministry Sergei Lavrov said it was not in Russia's interests for Ukraine to break up but added that Moscow wanted all citizens of the country to be given equal treatment by Kiev.
He also denied allegations that Russian agents had been fomenting unrest in eastern Ukraine and said he was seeking explanation from US of reports that CIA director John Brennan had visited Kiev.
Russian President Vladimir Putin's spokesman told Interfax he had been "receiving many appeals from eastern Ukraine... calling on him to help in some form", and that he was watching developments "with a great deal of concern".
At an emergency meeting of the UN Security Council on Sunday, Russia urged Kiev not to use force against protesters in eastern Ukraine.
The Russian Ambassador to the UN, Vitaliy Churkin, called on the government in Kiev to "start a genuine dialogue".
Mr Churkin warned UN diplomats that there were neo-Nazis and anti-Semites within the ranks of "the self-proclaimed government in Kiev".
He said the opinions and interests of protesters in eastern Ukraine had not been taken into consideration.
"The henchmen of Maidan [protest movement] must stop attacking their own people," said Mr Churkin.
But Ukraine's UN ambassador Yuriy Sergeyev said Moscow had artificially created the crisis in the east of the country.
Ukraine's foreign ministry said on Monday that it had proof Russian forces had been behind the "separatist operation" and said it would present its evidence at a meeting in Geneva later this week.
The assistant UN Secretary General had warned that Ukraine "teeters on the brink".
European Union foreign ministers meeting in Luxembourg to discuss the crisis approved measures to provide up to 1bn euros (£800mn; $1.4bn) of economic assistance to Ukraine.
The ministers also approved "the temporary reduction or elimination of customs duties" for Ukrainian exports to the EU.
Also on Monday, the US signed a $1bn loan guarantee agreement for Ukraine after a meeting between US Treasury Secretary Jacob Lew and his Ukrainian counterpart, Oleksandr Shlapak, in Washington.