SLOVYANSK, Ukraine — Ukraine’s security forces pressed their assault on pro-Russian militants in this separatist stronghold in eastern Ukraine on Saturday, even as the rebels freed seven European military observers held for more than a week as suspected NATO spies and the Kremlin cited the deaths of dozens of people in street violence in Odessa as proof that Ukraine could no longer protect its citizens.
As Ukrainian forces launched an assault on the eastern town of Kramatorsk, the degree to which events have spiraled out of the authorities’ control was evident in Odessa, where an official said 46 people had died in street violence and in a fire in a building held by pro-Russian militants. Until Friday, Odessa, a Black Sea port in southern Ukraine, hundreds of miles west of the restive eastern region, had been calm. Then pro-Ukrainian and pro-Russian groups clashed in the streets during the day, with the police apparently unable or unwilling to restore order.
Later Friday, the pro-Russian militants retreated to the local House of Unions, which was then set on fire as the two groups continued to battle. Most victims apparently were overcome by smoke or burned, but eight were said to have died leaping from the flames. The Odessa authorities said 214 people were injured in the various events, including 88 who were hospitalized. Three days of mourning were declared.
The death toll came from the Odessa regional prosecutor Ihor Borshulyak, who spoke to reporters on Saturday, the Interfax news agency said. He also announced that 144 people had been arrested and that one of several inquiries would look into whether the police had failed to execute their duties.
In Moscow, the Kremlin spokesman Dmitry Peskov said Russia would no longer be able to tell hundreds of ethnic Russians who he said were calling for help not to take up arms. “The authorities in Kiev are not only directly responsible, they are direct accomplices in these criminal actions,” Mr. Peskov said. “Their hands are full of blood.”
Mr. Peskov also said Russia regretted that the United States and the European Union had endorsed the military operations in southeastern Ukraine, saying they, too, bore responsibility for fueling more violence.
A day after two Ukrainian helicopters were reported to have been shot down as Ukraine moved to retake positions in and around Slovyansk, the authorities said the operation was continuing with an assault on the nearby town of Kramatorsk. “We are not stopping,” Interior Minister Arsen Avakov declared Saturday on his Facebook page.
Diplomats in Kiev confirmed that the European military observers — four Germans, a Czech, a Pole and a Dane — had been freed on Saturday morning. That followed the arrival of a Kremlin envoy, Vladimir Lukin, in Donetsk, the regional center south of Slovyansk.
Mr. Lukin and Thorbjorn Jagland, a Norwegian who is president of the Council of Europe, a Strasbourg-based organization that includes Russia and Ukraine and that monitors human rights, secured the release, the Council of Europe said in a statement.
The German-led team was detained on April 25 while carrying out a mission for the 57-nation Organization for Security and Cooperation in Europe, to which Russia, Ukraine and the United States belong. Last weekend, one observer, a Swede suffering from diabetes, was released, but the others were paraded before reporters in a spectacle that the German foreign minister, Frank-Walter Steinmeier, called “disgusting.”
Germany is seen as a key player in trying to resolve Ukraine crisis, and President Vladimir V. Putin of Russia, who values relations with Germany, had previously spoken of the need to release the observers. The degree to which the Kremlin holds sway over the militants holding the center of Slovyansk is not clear, but it clearly took Kremlin intervention to get the observers freed.
A spokeswoman for the self-declared authorities in Slovyansk would not comment on whether the militants still regarded the observers as NATO spies, what led to the resolution and whether any prisoners held by Ukraine’s government had been freed in exchange.
Mr. Peskov, the Kremlin spokesman, suggested that the events in Odessa meant that Russia could no longer urge ethnic Russians – who he said were calling the Kremlin by the hundreds for help – to relinquish their guns and rely on Ukraine for protection.
“From now on Russia, in fact, has lost its influence on those people, because it would be be impossible to persuade them to disarm amid a direct threat to their lives,” he said.
On Friday, Ukrainian military and police forces resumed their effort to retake Slovyansk, 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 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.
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 a bridge overlooking rail lines by the city’s southern border, 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 service member 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.
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.