Ukraine's state security service has said that pro-Russian separatists have placed explosives in a building they seized in the eastern city of Luhansk and are using weapons to hold around 60 people against their will.
Pro-Russian separatists occupied the SBU building on Sunday evening in one of a series of attacks in the east of the country.
At least nine people were injured when the separatists broke doors and windows throughout the building and raised the Russian flag.
"The anti-terror group of the security services of Ukraine (SBU) .. has established that the criminals have rigged the building with explosives ... and are holding around 60 people, threatening them with weapons and explosives," the SBU said in a statement.
"These actions are extremely dangerous and endanger the lives of people both inside and outside of the building," the statement said. "They are using terrorist measures."
However, protesters denied the charge that they had wired the building with explosives and were holding people against their will.
"There are no explosives, no hostages. We do not need hostages to get what we want," said Anton, one of the protesters who described himself as a coordinator of the action.
US Secretary of State John Kerry has accused Russian agents and special forces of stirring separatist unrest in eastern Ukraine, saying Moscow could be trying to prepare for military action as it had in Crimea.
Armed pro-Moscow protesters were still occupying Ukrainian government buildings in two cities in the largely Russian-speaking east.
However, police ended a third occupation in a lightning night-time operation.
The Ukraine government says the occupations that began on Sunday are part of a Russian-led plan to dismember the country, and Kerry said he feared Moscow might try to repeat its Crimean operation.
"It is clear that Russian special forces and agents have been the catalyst behind the chaos of the last 24 hours," he said in Washington, and this "could potentially be a contrived pretext for military intervention just as we saw in Crimea".
Moscow annexed the Black Sea peninsula last month after a referendum staged when Russian troops were already in control.
Earlier, Russian Foreign Minister Sergei Lavrov dismissed Western accusations that Moscow was destabilising Ukraine, saying the situation could improve only if Kiev took into account the interests of Russian-speaking regions.
Shots were fired, a grenade thrown and 70 people detained as Ukrainian officers ended the occupation in the city of Kharkiv during an 18 minute "anti-terrorism" action, the interior ministry said.
But elsewhere in Ukraine's eastern industrial heartland, activists armed with Kalashnikov rifles and protected by barbed wire barricades vowed there was no going back on their demand - a vote on returning to Moscow rule.
In the city of Luhansk, a man dressed in camouflage told a crowd outside an occupied state security building: "We want a referendum on the status of Luhansk and we want Russian returned as an official language."
The Kremlin's standoff with the West has knocked investors' confidence in the Russian economy, and the International Monetary Fund on Tuesday cut its forecast of growth this year to 1.3%, less than half the 3% it had originally projected.
Britain expressed fears that Russia wanted to disrupt the run-up to presidential elections next month in Ukraine, which has been ruled by an interim government since the overthrow of Moscow-backed president Viktor Yanukovych in February.
Ukraine, which was controlled by Moscow until the Soviet Union collapsed more than two decades ago, has been in turmoil since late last year when Yanukovych rejected closer relations with the European Union and tilted the country back towards Russia.
That provoked mass protests in which more than 100 people were killed by police and which drove Yanukovych from office, leading to Kiev's loss of control in Crimea.
In Kiev, Interior Minister Arsen Avakov partly pinned responsibility for the Kharkiv occupation on Russian President Vladimir Putin. "All this was inspired and financed by the Putin-Yanukovich group," he said.
An aide said police went in when the protesters failed to give themselves up and surrender their arms. Officers did not open fire, despite shooting and the grenade attack from the other side, he said. One police officer was badly wounded and some others less seriously hurt.
In Luhansk, a city of around 450,000, protesters have blocked streets leading to the state security building with barbed wire, tyres, crates, metal police barriers and sandbags.
Andrei, who said he had stormed the building on Sunday but would not give his family name, said the protesters had 200-300 Kalashnikovs and some stun grenades, but there had been no shooting so far.
"Once you've taken up arms, there's no turning back. We will stay until the authorities agree to hold a referendum on the status of Luhansk," he said.
A standoff also continued in the mining centre of Donetsk, Yanukovich's home base, where a group of pro-Russian deputies inside the main regional authority building on Monday declared a separatist republic.
Unlike in Kharkiv, there was no clear sign that further police operations were imminent in the other two cities. "We hope the buildings occupied in Donetsk and Luhansk will soon be freed," acting president Oleksander Turchinov said.
Russia has warned Kiev against using force to end the occupations but authorities may anyway have decided not to give Moscow an excuse to intervene, holding back in the hope that the protests will fizzle out.
White House warning
The White House warned Russian President Vladimir Putin against moving "overtly or covertly" into eastern Ukraine.
It said there was strong evidence that pro-Russian demonstrators in the region were being paid.
Mr Lavrov told Mr Kerry that constitutional reform was required to resolve the crisis.
Russia says this would give Ukraine’s regions more powers, as it believes the rights of ethnic Russians in Ukraine are being violated.
Moscow, meanwhile, said today it was ready to take part in talks with Brussels and Washington but insisted the ex-Soviet country's Russian-speaking east and south be represented in the negotiations.
"Indeed we are ready to consider a multi-party format in which the Europeans, the United States, Russia and the Ukrainian sides are represented," Russian Foreign Minister Sergei Lavrov told reporters.
"We are in favour of Ukraine's south and east being represented in such talks."