Resurgent government forces have hoisted the Ukrainian flag over pro-Russian rebels’ main stronghold after a devastating shelling onslaught that levelled much of the city but delivered Kiev its biggest success.

The self-proclaimed mayor of Slavyansk confirmed on Saturday that insurgents had abandoned the rustbelt city of 120,000.

Ukraine’s ability to win back Slavyansk – home to one of the country’s biggest weapons storage facilities that fell to the insurgents on April 6 – marks a key turning point in three months of low-scale warfare that has threatened the very survival of the ex-Soviet state.

Ukrainian Interior Minister Arsen Avakov said in a Facebook post the withdrawal was led by senior militia commander Igor Strelkov – alleged by Kiev to be a colonel in Russia’s GRU military intelligence unit.

Both Strelkov and Moscow deny any GRU link despite Western claims the Kremlin is covertly funding the uprising to destabilise Kiev’s new pro-European leaders and retain control over Russian-speaking eastern regions of Ukraine.

Ukrainian Defence Minister Valeriy Geletey told President Petro Poroshenko his forces had raised the national flag over city hall “in accordance with your order to liberate Slavyansk”.

Poroshenko stormed to victory in a May 25 election thanks to his vow to quickly resolve the country’s worst crisis since independence in 1991.

Most analysts think the 48-year-old chocolate baron desperately needed an early success to secure the trust of Ukrainians frustrated by their underfunded army’s inability to stand up to Russian aggression.

“The departure of the fighters was a surprise. Nobody was aware it was happening,” city resident Kolya Cherep said.

“This morning, I saw that there were no fighters in front of the town hall. Then I saw that there were none manning the barricades in town,” he said.

Strelkov himself had told the pro-Kremlin LifeNews channel on Friday his units “will be destroyed … within a week, two weeks at the latest” unless Russia helped secure an immediate truce.

The militia commander tweeted on Saturday that President Vladimir Putin’s repeated vow to use all “available means” to protect his compatriots in Ukraine now looked like an empty promise.

“They filled us with hope and abandoned us. Those were fine words by Putin about protecting the Russian people, defending New Russia. But only words,” Strelkov wrote.

Slavyansk is the symbolic heart of an uprising sparked by the February ouster of a pro-Kremlin administration in Kiev and fuelled by Russia’s subsequent seizure of Crimea.

Relentless artillery and sniper fire across a dozen blue-collar cities and towns have since claimed more than 470 lives and left Western leaders frustrated by repeated mediation failures.

Clashes in the economically-vital border regions of Lugansk and Donetsk picked up with renewed vigour when Poroshenko tore up a 10-day ceasefire agreement earlier this week.

His decision was immediately followed by the launch of a “massive” offensive by Kiev that prompted Germany and France to spearhead a new push for an immediate and lasting ceasefire.