KIEV, Ukraine -- With a fierce onslaught of gunfire and mortar shelling, Ukrainian government forces on Saturday expelled pro-Russian insurgents from Slovyansk, a long-blockaded rebel stronghold, government officials and separatist leaders said.
After rebels fled the city, the scene of some of the fiercest battles throughout the insurrection, government forces raised the blue-and-yellow Ukrainian national flag above the city council building.
While it was not clear that retaking Slovyansk signaled a decisive blow against the rebels, it showed Ukrainian forces finally gaining traction and reasserting state authority in eastern Ukraine, three months after separatists seized cities and towns throughout the region, fracturing this country of 45 million people.
"The state flag of Ukraine is proudly waving over the city, which militants thought was their impregnable fortress," President Petro Poroshenko declared in a statement. "It's not a complete victory and it's not a time for fireworks, but clearing Slovyansk of extremely well-armed bandits has a very symbolic meaning. This is a turning point in fighting militants for the territorial integrity of Ukraine."
The Ukrainian advance came four days after Mr. Poroshenko ended a cease-fire and ordered the military to resume efforts to crush the rebellion by force. On Tuesday, Ukrainian forces retook an important checkpoint at a border crossing with Russia, one of several seized by rebels and that the Ukrainian government and its Western allies said were used to allow Russian tanks, weapons and fighters to cross into the region.
There were no firm tallies of casualties though officials said that one government soldier had been killed and three wounded in fighting outside the city, and one separatist leader reported that about 150 rebels had sought medical help in Donetsk, the regional capital.
Ukrainian soldiers chasing rebels out of Slovyansk destroyed a tank, two combat vehicles and two armored personnel carriers, a spokesman for Ukraine's National Security and Defense Council, Andriy Lysenko, said.
Defiant rebel leaders confirmed that their fighters had fled under heavy attack by the Ukrainian military, but insisted that it was only a temporary setback. "You think you won?" Denis Pushilin, the parliament speaker of the self-declared Donetsk People's Republic, wrote on Twitter. "This was a tactical move. We will enter Slovyansk victoriously."
Andrei Purgin, an insurgent leader, said that rebels abandoned the city because they were overwhelmed. "What would you do if you were shelled with mortars and artillery guns and pounded from the air, and you had only three tanks and assault rifles?" he asked in an interview with the Interfax news service. "The Ukrainian security forces, in fact, tried to raze Slovyansk to the ground."
Government officials were jubilant. "Run!" the Ukrainian interior minister, Arsen Avakov, wrote in a Facebook post on the retaking of Slovyansk. "The terrorists are bearing losses, surrendering."
Ukrainian officials said that those fleeing included the well-known commander Igor Girkin, who the authorities here say worked for the Russian military's foreign intelligence directorate. In east Ukraine, he identified himself as Col. Igor Strelkov, which means shooter or gunman.
By midday Saturday, government troops were sweeping through neighborhoods of Slovyansk in search of any remaining fighters, officials said. Oleksandr Turchynov, the Ukrainian Parliament speaker, arrived at a checkpoint near Slovyansk, wearing military fatigues and body armor and spoke to a television crew as explosions could be heard in the background.
"As you hear," Mr. Turchynov said, "the anti-terror cooperation is going on, and units that take part in fighting the terrorists are going both east and south of Donetsk region. In parallel to that, the special forces units are controlling Krasny Liman to see if there are remnants of terrorist groups."
The separatist rebellion is the latest, and bloodiest, chapter in a crisis that began in November after Viktor Yanukovych, then Ukraine's president, rejected a trade accord he had promised to sign with the European Union, favoring, instead, closer ties with Russia. Protesters took to the streets of Kiev, and in February Mr. Yanukovych fled the capital. Within a week, Russia invaded Crimea, then quickly annexed the peninsula.
By early April, pro-Russian separatists were seizing public buildings, then entire towns and cities in the predominantly Russian-speaking east. President Vladimir Putin of Russia mobilized troops along the border, warning that he would use force if necessary.
Since then, the government has struggled to contain the insurgency, and diplomatic efforts to defuse the situation have largely failed.
With the West apparently reluctant to levy more sanctions on Moscow, Mr. Poroshenko declared last week that he was intent on ending the rebellion once and for all, though fears of a full-scale Russian invasion persist.
As the government reported its success in Slovyansk, the Ukrainian Orthodox Church on Saturday announced the death of Metropolitan Volodymyr, 78, the leader of the church's Moscow patriarchate who had struggled, through illness, to be a voice of conciliation in the political turmoil that has divided Ukraine and Russia.
Metropolitan Volodymyr's death could have broad political implications, setting the stage for a battle between clerics seeking closer ties with the Russian Orthodox Church, headquartered in Moscow, and those who want a more independent, distinctly Ukrainian church.
Ukraine is a deeply religious country and the overwhelming majority of the population identifies as Orthodox. The church, itself, however, is split, largely between the Moscow patriarchate, which is beholden to Patriarch Kirill I of the Russian Orthodox Church, a close ally of Mr. Putin's, and the Kiev patriarchate, which Moscow does not recognize.
Patriarch Kirill has long sought tighter control over the church in Ukraine, but a push to install a vocally pro-Moscow prelate could inflame a large segment of the Ukrainian public, which could view the appointment as a further effort by Mr. Putin to keep Ukraine within Russia's sphere of influence.
Slovyansk, once a quiet industrial city, was one of the first cities to come fully under rebel control after the start of the insurrection in April. Since early April, it was the site of numerous battles.
Local news services in eastern Ukraine reported that insurgents were traveling south from Slovyansk toward Donetsk, and that residents were fleeing the city fearing an imminent attack by the Ukrainian military.