SLOVYANSK, Ukraine -- The Ukrainian military landed airborne troops at an airport about 25 miles south of here Tuesday, raising tensions with Russia in the opening phase of what the Kiev government called a wider military operation to confront pro-Russian militants in the eastern part of the country.
Later in the day, a column of armored personnel carriers flying Ukrainian flags approached Slovyansk from the north, parking for a time beside a highway and setting up a checkpoint. Of all the cities in the east, Slovyansk seemed to have fallen most completely under the control of pro-Russian separatists, who have erected massive defensive barricades outside the buildings they occupy.
The Ukrainian authorities said the movements were the first in a campaign to drive separatists from government buildings in as many as 10 cities in eastern Ukraine. The initial steps suggested that the Kiev government, which had been hesitant to do anything to play into Moscow's narrative that Russian-language speakers are in need of protection, was now willing to use the military to try to restore order in some places.
In a conflict that has revealed deep East-West fault lines, the White House praised the move as a measured step toward restoring law and order, while Russian President Vladimir Putin in a phone call told United Nations Secretary-General Ban Ki-moon that he expected the international community's "clear condemnation" of the "anti-constitutional" operation. The Russian stock market fell by 3 percent on war jitters.
At the small military air base in the town of Kramatorsk, Ukrainian soldiers quickly took control, holding at bay separatists who had surrounded the field's perimeter, which is guarded by a barbed wire fence. At some point, the operation's commander, Gen. Vasily Krutov, left the base to speak to a crowd of protesters numbering about 500 to urge them to disperse. But the crowd remained hostile, and when he turned to head back to the base, he was roughed up, people in the crowd said -- shoved hard enough that his hat fell off. The crowd outside the air base then insulted and taunted the soldiers inside. "Guys, fly home; we didn't invite you here," one man yelled.
The situation, described by local reporters as a "mob scene," persisted into early evening, with the crowd occasionally surging forward through a gap in the fence and soldiers firing into the ground in front of those who approached too close. People in the crowd said at least two people were injured by the shots.
Russian television, which has consistently sought to play up grievances by pro-Russian activists who the West says are a tool of Russian intelligence, introduced its evening broadcast by announcing, "The illegal, criminal government in Kiev launches a war against its own people."
It repeatedly broadcast images of helicopters and a single warplane in the vicinity of the airport, where separatists had set up a roadblock outside the main entrance Sunday. The Russian news media, citing members of the armed opposition to the Kiev government, reported that several members of a pro-Russian militia had been injured at the airfield in firefights with the Ukrainian military. Later, it also reported four fatalities in the airport's vicinity.
The Russian Foreign Ministry released a statement Tuesday saying it was "deeply concerned" by the reports of casualties, which could not be independently confirmed and were denied even by pro-Russian activists at the airport.
The checkpoint the Ukrainian military established on a highway north of Slovyansk provided a further sign that the operation this time represented more than just words. Speaking to reporters following the armored column, Gen. Krutov delivered a sharp caution to any gunmen on the road ahead, saying, "They must be warned that if they do not lay down their arms, they will be destroyed."
While there were no credible reports of casualties or even any sustained firefights, Gen. Krutov later told The Associated Press that his forces had repelled a force of 30 men in green uniforms without insignia, shorthand for the unmarked Russian regulars who infiltrated and overran Crimea.
The airport protesters said in interviews that they felt no allegiance or kinship to the Kiev government and wanted to go their own way. "After Maidan, the east of Ukraine felt outside the political process," Vyachislav Filken, 47, a construction worker in the crowd, said, referring to the Kiev square where the uprising that led to a new government unfolded. "They wanted to put in their president and didn't ask us."
Mr. Filken said he wanted Ukraine to grant eastern regions autonomy, but that local people now felt emboldened and might demand simply to join Russia, as he said residents of Crimea had. "We want to live apart and be the bosses in our own land," another said.
After the operation began, masked gunmen at Slovyansk checkpoints prohibited civilian vehicles from leaving the city through a main checkpoint, citing Ukrainian military movements on the highway outside the city. Cars were allowed out by side roads.
Acting President Oleksandr V. Turchynov, the Ukrainian Parkliament speaker, said in Kiev that the operation "will be carried out in stages, and responsibly and in a balanced manner," adding, "The goal is the defense of citizens of Ukraine." Mr. Turchynov, who has asserted in recent days that Russian soldiers have joined the Ukrainian militants who have seized police stations and the entire town of Slovyansk, said the country was confronting a "colossal danger," but offered the assurance that "there will be no civil war."