(CNN) -- Ukrainian special forces moved against pro-Russian demonstrators in the eastern city of Donetsk late Monday after the country's acting President vowed to resist efforts to "dismember" his country, his office reported.
The troops cleared armed protesters from the headquarters of Ukrainian security services in Donetsk, one of three cities where pro-Moscow uprisings took place over the weekend, acting President Oleksandr Turchynov's office announced Monday night. There were no casualties in the operation, Victoria Sumar, deputy secretary of the National Defense and Security Council, told reporters.
Police battled protesters in one of the other two cities, while authorities set up a committee to negotiate with a self-declared "army" in the third.
The reports came several hours after Turchynov blamed "separatist groups coordinated by Russian special services" for the revolts, which he said echoed events leading to the Russian annexation of Crimea three weeks ago.
"Enemies of Ukraine are trying to play out the Crimean scenario, but we will not let this happen," Turchynov said in a televised message.
Pro-Moscow protesters seized government buildings, raised Russian flags and declared new governments in Donetsk, Luhansk and Kharkov on Sunday. Prime Minister Arseniy Yatsenyuk said the goal of the protesters was "to destabilize" Ukraine, allowing "foreign troops to cross the border and seize the territory of the country."
"We will not allow it," Yatsenyuk said.
Russia, which has an estimated tens of thousands of troops near the Ukrainian border, said it was "watching closely" and told Ukraine to quit blaming it for Ukrainian problems. President Vladimir Putin's government pushed Ukraine to set up a federal system in which regions with ethnic Russian majorities would have more autonomy, and its foreign ministry urged Ukraine to enter into talks over the issue.
"Ukrainian people want to get a clear answer from Kiev to all their questions. It's time to listen to these legal claims," a Foreign Ministry statement read. The Ukrainian government was acting "irresponsibly," it said.
But in Washington, U.S. officials urged Russian President Vladimir Putin's government to disavow the protests and warned further Russian intervention in Ukraine would bring stiffer economic sanctions than those already imposed over the Crimean annexation.
"If Russia moves into eastern Ukraine, either overtly or covertly, this would be a very serious escalation," White House spokesman Jay Carney told reporters. "We call on President Putin and his government to cease all efforts to destabilize Ukraine. And we caution against further military intervention."
In Donetsk, demonstrators took over a regional administrative building in the mining city on Sunday. Russia's state-run ITAR-Tass news agency reported that a self-proclaimed legislature, representing what it called the Donetsk People's Republic, had called a May 11 referendum on whether the area should join Russia.
The Russian news agency also said the Donetsk group asked Putin to send a "temporary peacekeeping contingent." The report could not immediately be independently verified.
Protesters also seized the regional administrative building in Kharkov on Sunday. Ukrainian Interior Minister Arsen Avakov, who was in the city, said on his Facebook page Monday that the building had been cleared and that public employees had returned to work.
But demonstrators returned Monday night to battle police, throwing Molotov cocktails and rocks and setting fire to barricades around the building, Ukrainian news outlets reported.
Police used water cannon and pepper spray to disperse the protesters late Monday. But about 50 others stormed the headquarters of a local television station, while others attempted to seize its tower to air Russian television channels, the Ukrainian news agency Ukrinform reported.
And in Luhansk, where demonstrators had taken over the offices of the security services and seized weapons, the regional government set up a working group to defuse the standoff peacefully. The group includes "authoritative public organizations and associations" in the region, as well as conflict-resolution experts, the regional government announced.
"The working group is ready for an open dialogue in order to return the life to a peaceful course, to restore the public order and to ensure the safety of citizens," it said.
Earlier, a group of armed, uniformed men in Luhansk declared themselves the "Army of the Southeast" and called on the population to turn out in support of "our rights and values."
"Within the last 24 hours, our army has increased many times," a spokesman for the group said in a video posted on YouTube. "We are ready to send a reserve unit to Donetsk if needed. But without the total people's support, we will not be able to change anything."
Donetsk is the hometown of ousted Ukrainian President Viktor Yanukovych, whose pro-Russian government was toppled in a popular revolt in February. Russia has called his removal a coup and dispatched thousands of troops to Crimea in support of a pro-Russian movement that seized power in that semi-autonomous Black Sea peninsula shortly after Yanukovych fled to Russia.
That new Crimean administration held a referendum on independence that passed overwhelmingly, despite being condemned as unconstitutional by the new Ukrainian government and by Western powers. Within days, the territory was annexed by Russia.
In mid-March, Yatsenyuk warned that demonstrations by what he called "political tourists" with foreign passports were under way in the east and warned that Russia might attempt to follow the model used to wrest Crimea from Ukraine.
Ukrainian officials are still evacuating military installations in the territory, and Kiev's defense ministry reported that a Ukrainian naval officer was shot and killed by a Russian soldier on Sunday as the officer prepared to leave.
Russia based its intervention in Crimea on allegations that the territory's Russian-speaking majority was being mistreated. Carney said the claim "bore no resemblance to the actual truth at the time."
U.S. officials have said Russia had about 40,000 troops near the Ukrainian border. Putin told German Chancellor Angela Merkel last week that he'd ordered a withdrawal of some Russian troops from his country's border area with Ukraine, Merkel's office said.
But NATO later said it saw no sign that Russia was pulling its forces back from the border. And a small number of additional Russian troops are believed to have moved toward Russia's border with Ukraine, essentially negating any previous withdrawal, two U.S. officials told CNN on Monday.
The European Union's foreign policy chief, Catherine Ashton, said the EU was monitoring the situation in eastern Ukraine closely with concern.
"We reiterate our call for de-escalation and the avoidance of further destabilization of Ukraine," she said.
CNN's Marie-Louise Gumuchian, Alla Eschenko and Lindsay Isaac contributed to this report.