DONETSK, Ukraine -- Ukrainian authorities moved to quell pro-Moscow uprisings along the Russian border with mixed results Tuesday, retaking one occupied regional headquarters and watching protesters consolidate their hold on another.
In a third city, Luhansk, Ukraine's Security Service said separatists armed with explosives and other weapons were holding 60 people hostage inside the agency's local headquarters.
Those occupying the building issued a video statement saying they want a referendum on the region's status and warning that any attempt to storm the place would be met with armed force.
In the video, posted by Ukrainian media, a masked man identified the occupiers as Ukrainian veterans of the Soviet war in Afghanistan and said that if authorities try to retake the building, "Welcome to hell, then!"
The Ukrainian government and the U.S. have accused Moscow of fomenting the unrest as a pretext for another Russian military incursion like the takeover of Crimea last month. Up to 40,000 Russian troops are massed along the Ukrainian border, according to NATO.
All the cities affected by the uprisings are in Ukraine's industrial heartland in the east, which has a large population of ethnic Russians and where hostility is strong toward the government that took power in February after the ouster of Kremlin-friendly President Viktor Yanukovych.
European Union envoy Catherine Ashton said she will meet with U.S., Russian and Ukrainian foreign ministers next week to discuss the situation -- the first four-way meeting since the crisis erupted.
In Washington, Secretary of State John Kerry threatened tougher economic sanctions against Moscow.
"What we see from Russia is an illegal and illegitimate effort to destabilize a sovereign state and create a contrived crisis with paid operatives across an international boundary," Kerry told the Senate Foreign Relations Committee.
Kerry called the demonstrations in eastern Ukraine a "contrived pretext for military intervention just as we saw in Crimea."
Earlier Tuesday, Ukrainian authorities battled pro-Russian protesters and regained control over a government building in Kharkiv, the country's second-largest city, evicting the protesters and detaining dozens.
Acting President Oleksandr Turchynov told Parliament that several police were injured during the Kharkiv clashes with what he termed separatists.
In Donetsk, a city 250 kilometers (155 miles) south of Kharkiv, protesters dug in for their third day at the 11-story regional administration headquarters they captured on Sunday and began forming their own parallel government.
Serhiy Taruta, the governor of Donetsk, scoffed at the shifting events in his city.
"I call this a theater of the absurd," he said. "It is just artists performing, but the main thing is that there is an ever-dwindling audience."
There was little sign Tuesday afternoon that Ukrainian government forces had any immediate plans to clear the regional administration building, and Taruta insisted he wanted to see the situation resolved peacefully.
The city has been the site of weekly rallies and marches, but Sunday saw an escalation of that strategy when masked men carrying batons burst through police lines to take over the building. By Tuesday, 6-foot walls of car tires wrapped in razor wire had been erected against any attempt to storm the place.
On Monday, the demonstrators declared the creation of a sovereign Donetsk Republic and called for a referendum on the issue to be held no later than May 11.
Despite claims by the demonstrators to represent the entire Donetsk, a region of more than 4 million people, rallies outside the administration building since the weekend have drawn crowds of only a few thousand.
While none of the leading figures in the self-proclaimed Donetsk Republic movement have said they want the region to join Russia, they have declined to rule out the option. Their initial priority, they say, is to secure autonomy, after which the population will be asked whether it wishes to become part of Russia.
White House spokesman Jay Carney said this week there was strong evidence some pro-Russian protesters in eastern Ukraine were hired and were not local residents.
People in the corridors of the Donetsk building -- many of them wearing masks and carrying sticks -- invariably identified themselves as being from Ukraine when questioned.
Similarly, representatives of the Donetsk Republic movement dismissed claims of Russian support, saying volunteers had arrived from all across the Donetsk region.
"I live in Mariupol, I have three children there, and I came here so that my children could have a normal life in the future," said Sergei Renin.
Renin said he was proud to be a Ukrainian but concerned over what he described as attempts by the "Nazi government" in Kiev to trample the rights of Russian speakers.
Even Taruta expressed doubts that Russia was orchestrating the unrest. Still, authorities have said that overwhelming numbers of Russian citizens were visiting Ukraine on recent weekends that saw spikes in anti-government rallies.
The Kremlin has pushed for constitutional reform in Ukraine that would turn the country into a federation, with broad powers for each of its regions. The demands reflect Russia's desire to maintain influence over its neighbor and prevent Ukraine from joining NATO.