Ukrainian authorities have warned they are prepared to use force to clear several government buildings seized by pro-Russian separatists in the east of the country.
Wednesday's warning came as protesters continued to occupy the headquarters of Ukraine's Security Service in the eastern city of Luhansk.
Hundreds of supporters camped outside and shouting "Putin! Putin!" in support of the Russian President, the Associated Press news agency reported.
Arsen Avakov, the Ukrainian interior minister, said the standoff in Luhansk and the two neighbouring Russian-leaning regions of Donetsk and Kharkiv must be resolved within the next two days.
"I want to repeat that there are two options: political settlement through negotiations and the use of force," Avakov told reporters. "We are ready for both options."
Avakov was speaking as anti-government protesters in Luhansk erected high barricades along a thoroughfare running in front of the security service premises.
The Security Service had earlier said that the separatists inside the building, armed with explosives and other weapons, allowed 56 hostages to leave the building during the night. A spokeswoman said there were no other hostages.
But Tetyana Pohukay, a regional police spokeswoman, disputed that statement, saying there had never been any hostages inside, according to the Interfax news agency.
The Luhansk security services building was among several government offices seized by pro-Moscow groups on Sunday in an escalation of protests against the interim government in power since President Viktor Yanukovich was forced to quit in February after months of protests against his decision to ditch an EU trade pact in favour of closer ties with Moscow.
Serhiy Tyhipko, a lawmaker associated with the previous Ukraine government, urged the authorities not to storm the building in Luhansk, calling for a negotiated peaceful solution.
Now, with this ultimatum, it will be a question of what kind of deal can both sides come to. Both protests and police are very keen to avoid any blood shed,
Tyhipko said the protesters were demanding to turn Ukraine into a federal state with broad regional autonomy, not to secede.
"The people are not bringing up the issue of breaking off from Ukraine and are not calling for the help of foreign countries,'' Tyhipko said on his Facebook page.
Turning Ukraine into a federation is Russia's key demand and the new government in Kiev has refused to fulfill it, calling it a precursor to a break-up.
In Donetsk, where protesters were still occupying the government building, the regional governor was meeting with key figures in the pro-Russian protest movement to try to find a solution to the crisis.
"The ultimatum made put the pressure on people here in Donetsk," Al Jazeera's Kim Vinnell said, reporting from the eastern region.
She added that negotiations were under way between both separatists and police in Donetsk and Luhansk.
"Now, with this ultimatum, it will be a question of what kind of deal can both sides come to. Both protesters and police are very keen to avoid any bloodshed," she said.
All the cities affected by the uprisings are in Ukraine's industrial Russian-speaking heartland in the east, which has a large population of ethnic Russians and where economic and cultural ties to Russia are strong, according to AP.
Many residents are suspicious of the government that took power in February.
Overnight, speakers at a gathering in front of the building condemned the government in Kiev and renewed demands to be allowed to hold a referendum on declaring autonomy for their region.
That demand is similar to one that preceded Crimea's annexation by Russia.
Meanwhile, Russia has threatened that its gas company Gazprom could ask Ukraine to pay for gas in advance, according to Russian state news agencies.
In a meeting on Wednesday with government ministers, Putin said that asking for advance payments "corresponded with the contract" between Ukraine and Moscow. But he asked that Gazprom refrain from such drastic measures until "additional consultations" between both sides.