SLOVYANSK, Ukraine — Two days after Ukraine’s interim government declared itself all but helpless to control events in the eastern part of the country, Ukrainian military and police forces on Friday resumed their effort to retake this rebel-controlled city, forcing armed separatists from the city’s outskirts with armored vehicles, helicopters and ground troops — and seemingly pushing ever closer to a confrontation with Moscow.

The assault began before dawn, and by early morning church bells sounded a general alarm. With helicopters flying along the city’s edge, troops and armored personnel carriers approached the city from multiple sides. The fighting was brief and then sporadic, however, and clashes subsided by early afternoon, before resuming at night. When movements stopped the Ukrainian troops had advanced, capturing and holding separatist checkpoints and posting infantry and armored vehicles on a bridge overlooking rail lines by the city’s southern border.

Violence also erupted Friday in the previously calmer port city of Odessa, on the Black Sea, where dozens of people died in a fire related to clashes that broke out between protesters holding a march for Ukrainian unity and pro-Russian activists. The fighting itself left four dead and 12 wounded, Ukraine’s Interior Ministry said. Ukrainian and Russian news media showed images of buildings and debris burning, fire bombs being assembled and men armed with pistols.

 

 

 

 

 

 

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The deaths expand the increasingly violent struggle for control over Ukraine’s Black Sea port, which had been quiet until last week, when seven people were wounded in a roadside bombing.

The government’s actions in Slovyansk drew an immediate and indignant protest from the Kremlin, which said Ukraine had effectively destroyed a plan negotiated last month with the United States, Russia, Ukraine and the European Union intended to de-escalate tensions.

Blaming the authorities in Kiev, Ukraine’s capital, a spokesman for President Vladimir V. Putin of Russia, Dmitri S. Peskov, told Russian news agencies that “the Kiev regime ordered combat aircraft to fire at civilian towns and villages, launching a punitive operation.”

Moscow repeated its warning that it reserved the right to intervene to protect its interests and Russian-leaning residents of eastern Ukraine. But there were no signs of an imminent move across the border.

The Russian ambassador to the United Nations, Vitaly I. Churkin, speaking in New York, described Ukrainian military operations as “punitive” and said there were English-speaking foreigners among what he called “ultranationalist groups” in Slovyansk. He accused his Western rivals of double standards, noting that while they had called on Ukraine’s former president, Viktor F. Yanukovych, to exercise restraint, they condoned the use of force by the current leaders in Kiev.

The United States ambassador, Samantha Power, accused Russia of spreading lies, and described Ukraine’s reaction to Russian provocation as “reasonable.”

“The Ukrainian people and government have embarked on an effort today to reclaim one city in the eastern part of their country,” she said. “Their response is reasonable, it is proportional, and frankly it is what any one of our countries would have done in the face of this threat.”

The Ukrainian army units in Slovyansk did not seem eager to engage the militants fully, and appeared to limit their activities for the day to tightening a cordon around the militant stronghold. The city’s center remained under the control of antigovernment militias, who manned barricades as streets fell nearly silent ahead of what residents feared could become a general assault.

“They are coming at us from all sides,” said one fighter in camouflage and sneakers, who gave his name as Sergei, and who held a Kalashnikov assault rifle and said he was a veteran of the Soviet Army.

Both the government and the separatist forces said two helicopters were brought down in the fighting, with at least one crew member killed. These reports could not be verified, and hours later the wreckage had not been found by independent observers.

Late at night, the Ukrainian government said two members of an airborne brigade had been killed, apparently in the brief but intense evening clash at the bridge, which by day had been held by airborne troops.

Earlier, as the first round of fighting died down, Ukrainian troops were posted at their newly captured positions in the villages of Bylbasovka and Andreyevka on the city’s perimeter, where residents flocked to argue with them and urge them not to fight.

In Bylbasovka, a Ukrainian serviceman who identified himself as a staff officer for one of the battalions participating in the operation stood with troops facing about 75 angry residents who demanded that they leave.

“We came to prevent further destabilization of the situation,” said the officer, who gave a first name, Vitaly. “We have nothing against peaceful citizens.”

The residents argued with the troops standing at the front rank, at one point chanting, “Shame! Shame! Shame!” Three more busloads of troops stood behind sandbags and watched warily.

The scene was similar at the bridge in Andreyevka, though on a larger scale. Roughly 10 Ukrainian armored personnel carriers and a few trucks had captured the bridge, where they were met by 200 or more unarmed residents who came out to defy them or urge them to defect.

The front rank of troops faced verbal abuse but held its position, looking weary and under stress.

One woman scolded the soldiers unrelentingly. “You came with weapons and tanks!” she shouted, her face only a few feet from theirs. “What, you can’t talk to us like normal people?”

Nearby, a man demanded to know who had ordered the operation. “Who called you here?” he asked.

“There are illegal weapons being used here,” the soldier replied.

As the day progressed, residents created new checkpoints, many that appeared under the control not of militias but of angry citizens with wooden rods.

There were signs as well that the dark undercurrents of suspicion that can accompany civil war had gripped some of the populace and the militias.

At one checkpoint, a man who had arrived on foot to watch the fighters was angrily ordered at gunpoint against a fence facing the street and was handcuffed to a wall. One militia member punched him twice in the stomach, and another slapped his head and clapped his hands over his ears.

The fighters said they had seen and searched the man the day before, when he was found with no money and no phone.

Now, on a day of fighting and with the Ukrainian Army at the city’s edge, he had reappeared and been found with a new cellphone and about $350 worth of Ukrainian currency — evidence, they said, that he was an informant. One fighter went through recent activity on the man’s phone, at one point striking him to speed up an answer. “He is a spy,” said another fighter, Dima.

The fighters left the man handcuffed to the wall for nearly four hours, the last 45 minutes in drenching rain, before abruptly returning his phone and his money and releasing him.

Much of eastern Ukraine slipped beyond the control of the authorities in Kiev as pro-Russian militants began taking control of a string of official buildings more than three weeks ago. The separatists have also captured a German-led team of military observers affiliated with the Organization for Security and Cooperation in Europe. Both sides gave competing accounts about the fighting on Friday, agreeing only on the unverified claim that at least two Ukrainian helicopters had been hit by ground fire. The Defense Ministry in Kiev said two Mi-24 attack helicopters had been shot down, killing at least two airmen.

The Ukrainian domestic intelligence agency, S.B.U., said that one helicopter had been brought down with a heat-seeking, shoulder-fired missile, and suggested that the presence of such a weapon in the conflict showed the separatists had outside support.

By night in Ukraine, neither side had shown clear evidence of heat-seeking missiles or downed aircraft, although one pro-Russian television station showed footage of what it said was a captured pilot.

Vyachislav Ponomaryov, the self-appointed mayor of Slovyansk, said four or five members of the city’s antigovernment militias had been killed, and at least three others wounded. His figures could not be corroborated.