Ukraine's interior minister has warned pro-Russian activists who have taken over state buildings in eastern cities to enter talks to find a political solution or face "force".
Arsen Avakov said the situation would "be resolved in 48 hours" either way.
Earlier, some of those who had been inside security service offices in Luhansk since Sunday left the building.
The EU, Russia, US and Ukraine are to meet next week in the first four-way meeting since the crisis erupted.
The talks are aimed at breaking the impasse since Russia annexed the southern Ukrainian peninsula of Crimea in February. Russian troops are now massed along the borders of the two countries.
In another development on Wednesday, Russian President Vladimir Putin suggested state-controlled energy company Gazprom should make Ukraine pay up front for its gas, but said it should consult with Kiev first.
Ukraine, which is on the verge of bankruptcy, owes Gazprom $2.2bn (£1.3bn) for natural gas supplies and recently missed a payment deadline.
Tensions have been high since government buildings in the eastern cities of Luhansk, Donetsk and Kharkiv were taken over by pro-Russian activists on Sunday.
Mr Avakov said an "anti-terrorist operation" was under way in the three regions and would be concluded within the next two days.
"There are two options," he told journalists, " political settlement through talks or the use of force.
"For those who want dialogue, we propose talks and a political solution. For the minority who want conflict they will get a forceful answer from the Ukrainian authorities."
The leader of the armed activists inside the state security service building in Luhansk has appealed to Russia's President Vladimir Putin for help.
Calling Luhansk the "last remaining hope for all Ukraine", the man identified only as Vitaly said: "Mr Putin, have mercy on your fighters. If you lose us then you will lose the last hope to create a good neighbour".
Earlier, Ukraine's security service said 56 people held inside its Luhansk offices had been allowed to leave following two rounds of negotiations with local politicians.
On Tuesday, it said "radicals" were armed and holding 60 people against their will. It is not clear exactly how many people remain in the building.
During a rally outside the building overnight, speakers condemned the interim leadership in Kiev and repeated their call for a referendum on greater regional autonomy, the Associated Press reports.
Ukraine's authorities said on Tuesday they had retaken control of the government building in Kharkiv.
Protesters in the regional authority building in Donetsk have been urged to leave. MP Nikolai Levchenko told reporters he feared for the safety of local residents.
"This should be resolved peacefully, he said. "I will do my best to protect them, even if they are wrong. But the most important thing is to secure the other citizens."
Barricades of barbed wire, tyres and even car bumpers surround the buildings.
A Russian flag flies by a barricade erected beside the security service building in Luhansk
These activists man the barricade outside a government building in Donetsk
Car bumpers have been used - along with tyres and barbed wire - to build barricades outside government buildings
Police officers' protective shields lie close to administrative buildings in Kharkiv
Moscow has warned Ukraine that using force to end the protests could lead to civil war.
On Wednesday, German Chancellor Angela Merkel criticised Russia for not doing enough to ease the tension.
She told parliament: "Unfortunately, in many areas it is not clear that Russia is contributing to a de-escalation of the situation."