Commandos engaged in gunfights with men who had set up roadblocks and stormed a Ukrainian police station in Slovyansk, and at least one officer was killed, Ukrainian officials said.
Several officers were injured in the operation, as were four locals, the officials said.
Russian news media and residents here disputed that account, saying the Ukrainian forces had only briefly engaged one checkpoint.
In either case, the central government in Kiev has turned to force to try to restore its authority in the east, a course of action that the Russian government has repeatedly warned against.
With tens of thousands of Russian troops massed along Ukraine’s eastern border near Donetsk, Western leaders have worried that Moscow might use unrest in Ukraine’s mainly Russian-speaking areas as a pretext for an invasion.
Both governments intensified their statements on Sunday.
Ukraine’s interim president, Oleksandr V. Turchynov, issued another ultimatum, saying separatists should vacate occupied buildings by Monday or face a “large-scale antiterrorist operation” that would include the Ukrainian military.
And Russia claimed that the Ukrainian government was cracking down at the behest of American and European officials.
Ukraine’s ousted president, Viktor F. Yanukovych, speaking late Sunday in Rostov-on-Don, in Russia, echoed Moscow’s charges of American meddling.
Insisting that he remained Ukraine’s commander in chief despite having fled to Russia more than a month ago, he ordered Ukrainian troops to defy what he called “criminal orders” for a crackdown and said the country stood “on the brink of civil war.”
The police station contested by Ukrainian forces was one of several security centers in the eastern region of Donetsk that were seized on Saturday by masked gunmen in coordinated raids that the Ukrainian authorities denounced as Russian “aggression.”
By Sunday afternoon, the government’s push to reassert its authority in a vitally important industrial and coal-mining region appeared to have made little headway.
Pro-Russian protesters appeared to control not only the police station but also the entire town of Slovyansk, having set up checkpoints at major streets leading into town.
The protesters blocked a major highway in the east, and flags of Russia and their newly declared and unrecognized People’s Republic of Donetsk flew over administrative buildings in several other midsize towns.
These included Mariupol, where protesters seized a building Sunday.
Roman Svitan, a security adviser to the Ukrainian authorities in Donetsk, said the operation on Sunday was carried out by Alfa, a special services unit of Ukraine’s state security service.
He gave an upbeat assessment of its progress, saying Ukrainian forces had evicted gunmen from the Slovyansk Police Headquarters, though protesters there said nothing of the sort had happened.
Mr. Svitan said most of the expelled gunmen were local pro-Russian extremists, but they had also included Russian operatives.
Residents and men standing by barricades in Slovyansk denied that Ukrainian forces had even entered the town on Sunday.
They said one local man who had been out fishing was in a hospital with a wound from a shooting on a highway outside town.
Russian television and some locals said the Ukrainian nationalist group Right Sector had attacked protesters at a checkpoint, injuring the fisherman.
Requests to speak to a leader of the armed men produced a man wearing a ski mask who introduced himself as Aleksandr and described himself as a deputy commander of the city of Slovyansk after its merger with the People’s Republic of Donetsk.
He gave a different account of the circumstances behind the wounding of the fisherman, saying he was struck by Ukrainian armored personnel carriers that opened fire on a barrier made from a pile of tires on the edge of town, then drove away.
“Our guys took cover, and the shooting stopped,” he said.
Ukrainian helicopters buzzed over the town around noon, but no soldiers were seen.
At one barrier, pro-Russian protesters felled trees across a road into town, guarded by men in ski masks carrying military rifles.
Russia’s Foreign Ministry issued a blistering denunciation of the Ukrainian government.
In a statement on the ministry’s Facebook page, the Russian government accused the Kiev authorities of threatening violence “against anyone who does not agree with the nationalist-radicals, chauvinistic and anti-Semitic actions” in Kiev that, it said, were being carried out “with direct support from the United States and Europe.”
At Russia’s request, an emergency meeting of the United Nations Security Council was held Sunday night.
Security Council members traded competing narratives of what was happening in eastern Ukraine, as Russia’s envoy, Vitaly I. Churkin, echoed the views of the Kremlin and his Western rivals deplored what they called Russian propaganda.
Mr. Churkin called on world leaders to condemn the “henchmen of the Maidan,” a reference to Independence Square in Kiev, where the uprising that led to the president’s ouster unfolded.
The British ambassador, Mark Lyall Grant, pointed the finger at Moscow.
“What we are witnessing is a well-orchestrated campaign to destabilize the country,” he said.
In Washington, the State Department took the unusual step of issuing a “fact sheet” alleging that Russian officials had made 10 false claims about the crisis in Ukraine.
“Russia continues to spin a false and dangerous narrative to justify its illegal actions in Ukraine,” the State Department said.
“We would not be seeing the violence and sad events that we’ve witnessed this weekend without this relentless stream of disinformation and Russian provocateurs fostering unrest in eastern Ukraine.”
According to the State Department, more than a dozen Russian intelligence agents have been arrested by the Ukrainian government in recent weeks, contradicting Moscow’s assertion that its agents are not active in Ukraine.
Russian news media reported Sunday that the American C.I.A. director, John O. Brennan, had arrived in Kiev on a secret visit to advise Ukrainian officials in charge of domestic security and defense.
The American Embassy in the Ukrainian capital declined to comment on the reports.
One protester who gave his name only as Ivan said the town had revolted because the new government in Kiev disregarded weeks of what he called peaceful protests demanding a referendum to secure greater local autonomy and establish Russian as a second official language.
The authorities in Kiev offered both last week, but the effort did little to resolve the standoff.
The unrest in Donetsk, eastern Ukraine’s most populous region, began April 6 in the regional capital when pro-Russian activists seized government headquarters and declared the People’s Republic of Donetsk.
According to the Ukrainian Interior Ministry, 400 Makarov handguns and 20 automatic weapons were looted on Saturday from the police station in Slovyansk.
“The goal of the takeover was the guns,” the ministry said.
By early evening Sunday, the city was gripped by fear after two unexplained shootings of cars, and as men with guns set up improvised checkpoints.
The men asserted they were locals, and nothing suggested otherwise:
Some wore mismatched camouflage, but most were out in jeans and ski jackets.
Some appeared to be drunk.
One of the cars that were shot at, a silver Renault, had veered into a curb after it was struck, with three bullet holes on the driver’s side.
Older women who had gathered nearby looked on in shock at the scene of mayhem.
They said the occupants had tumbled out and tried to run, but were then shot on the sidewalk.
One person died and two were injured, these witnesses said. In the other shooting, a bullet hit a taxicab, but nobody was hurt.
Ukrainian police said at least two people had been wounded in shootings in the city.
Source: The New York Times