Ukraine ordered the complete withdrawal from the ruined city of Avdiivka in eastern Ukraine before dawn on Saturday, surrendering a city that had been a military stronghold for the better part of a decade, in the face of withering Russian bombardment and relentless assault.

“Based on the operational situation around Avdiivka, in order to avoid encirclement and preserve the lives and health of servicemen, I decided to withdraw our units from the city and move to defense on more favorable lines,” Gen. Oleksandr Syrsky, Ukraine’s top military commander, said in a statement issued overnight.

The fall of Avdiivka, a city that used to be home to some 30,000 people but is now a smoking ruin, is the first major gain Russian forces have achieved since May of last year. In recent weeks, Russian forces have been pressing the attack across nearly the entire length of the 600-mile long front.

Ukrainian forces had begun withdrawing from positions in the southern part of the city on Wednesday. They have been engaged in a desperate battle to avoid encirclement inside the city for several days, as Russian forces advanced from multiple directions.

Oleksandr Tarnavskyi, the head of Ukraine’s forces in the south, said there had been no choice but to withdraw, given the Russian advantage in firepower and the number of soldiers they were willing to throw into the battle.