WATCH: Armed separatists maintain grip on government building in Slovyansk as Kyiv deadline passes.
Ukraine's acting President Oleksandr Turchynov has said that the Ukrainian government is "not against" a national referendum on the country's form of government.
In televised comments on April 14, Turchynov said a referendum could be held at the same time as presidential elections scheduled for May 25.
Turchynov said he was sure the majority of Ukrainians would back a united, independent country.
In Moscow, Russian Foreign Minister Sergei Lavrov said more information was needed on the referendum mooted in Kyiv. He said Ukraine's Russian-speaking eastern regions should be involved in drafting the constitution that should be put to a referendum.
The holding of a referendum on broad local rights or eastern Ukraine possibly joining Russia has been one of the key demands of pro-Russia separatists who have occupied state buildings in the east of the country. The Ukrainian-speaking west of the country rejects federalization.
There were no signs the separatists were preparing to comply with a government ultimatum to hand over their weapons and leave the state buildings by early morning on April 14.
On April 14, at least 100 separatists seized police headquarters in Horlivka, the latest town in the east where armed separatists have stormed government buildings.
WATCH: Protesters storm police headquarters in Ukraine's Horlivka
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Protesters Storm Police Headquarters In Ukraine's Horlivka
Turchynov has also invited the United Nations to join the Ukrainian army in an operation against separatists in the east.
In a telephone conversation with Secretary-General Ban Ki-moon, Turchynov suggested that an "antiterrorist operation" could be conducted jointly by Ukrainian security forces and UN peacekeepers, according to the presidential website.
WATCH: Separatists seize a police building in Kramatorsk on April 13.
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Armed Separatists Assault, Capture Police Station In Kramatorsk, Ukraine
The government had threatened a "large-scale" military operation if the separatists failed to comply with the deadline it set.
Turchynov said on April 14 that an antiterrorist operation involving the army would begin in eastern Ukraine, and that the Donbas region "will soon be stabilized."
Speaking at a news conference on April 14, Lavrov, said that those who encourage the actions of Ukraine's government -- which Moscow considers illegitimate -- should bear responsibility.
He said the use of force against pro-Russia protesters would undermine prospects for a planned four-way meeting on April 17 involving Ukraine, Russia, the United States, and the European Union.
Lavrov said it was not in Russia's interests for Ukraine to break up, and denied Ukrainian and U.S. allegations that Russian undercover agents were behind the separatist unrest.
He also said he was seeking an explanation of reports that the director of the CIA, John Brennan, had visited Kyiv.
Ukraine's ousted President Viktor Yanukovych speaking on Russian television on April 13 claimed that Brennan had met with Ukraine's new leadership and "in fact sanctioned the use of weapons and provoked bloodshed."
Yanukovych fled to Russia after he was ousted in late February.
EU foreign ministers are meeting in Luxembourg later on April 14 to discuss the EU's response to the escalation of the Ukraine crisis.
Also on April 14, the Standing Committee of the Organization for Security and Cooperation in Europe (OSCE) is to hold an extraordinary meeting to debate the situation in eastern Ukraine.
With reporting by Reuters, AFP, Interfax, and UNIAN
