Ukrainian forces entered the key Russian military stronghold of Izium on Saturday, continuing their rapid advance across the northeast and igniting a dramatic new phase in the more than sixmonth war.

“Izium was liberated today,” the city’s mayor, Valeriy Marchenko, said in an interview.

While he was not yet in the city himself, he said that he was in contact with the police and that emergency services were working to clear it of possible hazards before residents could return.

Russia’s ministry of defence — which a day earlier had said that it was moving to reinforce its defensive positions in the region — confirmed on Saturday that it had pulled its forces out of Izium, six months after its forces laid siege to and then seized the city.

In a statement, it presented the retreat as a pre-planned move, intended to strengthen its efforts in the east where its army has been bogged down for weeks.

Maintaining control of towns and cities has at times proven tenuous over the course of the war, and it was not immediately clear how secure Ukraine’s control over Izium was and what efforts Russia might take to try to win it back.

But the loss of Izium — a strategically important railway hub that Russian forces seized in the spring after a bloody weeks-long battle — could mark a turning point in the war, dwarfed only by Russia’s humiliating defeat around the Ukrainian capital, Kyiv, in the spring.

The first signs that Russian forces would retreat rather than fight emerged late on Friday.

“Yesterday evening, Russians put a white flag nearby the railway station,” Yevhen, a Ukrainian officer who participated in the liberation of Izium, said in a telephone interview.

“There was street fighting all over the night.”

He asked to be identified by only his first name out of concerns for his security.

Much about the Ukrainian offensive in the Kharkiv region, where Izium is, was shrouded in uncertainty amid a lack of official confirmation, and military analysts cautioned that it was a fast-moving situation that could change by the hour.

But the lightning offensive in the country’s northeast has reshaped what had become a grinding war of attrition.

In a matter of days, Russian front lines have buckled, Moscow’s troops have fled and one village after another has come once more beneath Ukraine’s yellow and blue banner — like the town of Kupiansk just north of Izium, which sits on key supply routes to the eastern front line.

Ukraine’s Security Service posted a photo on Telegram showing members of the special forces in Kupiansk. “We move further!” the post read, according to the Ukrinform news agency.

New York Times News Service