Russian troops have been relentlessly bombarding the city, which is the administrative center of the Luhansk region, and the Ukrainians are retreating to avoid encirclement.
Russia controls around 95% of the Luhansk province and around 50% of the neighboring Donetsk province. Both are in the disputed Donbas region.
Haidai said the defenders will continue fighting from new positions after they retreat.
The Ukrainians had fought Russians on the streets of the city before holing up in a chemical factory.
In Mariupol, the last remaining Ukrainian fighters before the city was taken by Russia had hidden inside a steel plant before being removed and taken away as prisoners of war in May.
Russian troops have made gains in Sievierodonetsk and neighboring Lysychansk as it wears down Ukraine’s resistance.
“We will have to pull back our guys,” Haidai said. “It makes no sense to stay at the destroyed positions, because the number of casualties in poorly fortified areas will grow every day.”
Russia’s defense ministry claimed Friday it had “fully blocked” four Ukrainian battalions and a foreign mercenary unit – around 2,000 soldiers – south of Lysychansk.
Haidai said the Russians were also advancing toward Lysychansk from Zolote and Toshkivka, adding that Russian reconnaissance units conducted forays on the city edges but were driven out by its defenders.
Russia invaded Ukraine in what it called a “special military operation” four months ago on Feb. 24 and has more recently shifted its focus to the Donbas region following its failure to capture the country’s capital, Kyiv.
U.S. and allied leaders have made visits to Kyiv to meet with Ukrainian President Volodymyr Zelenskyy in a show of support – the leaders of France, Italy, Germany and Poland visited last week – and the U.S. and its allies continue to support the country with military aid.
The U.S. most recently sent medium-range American rocket launchers to Ukraine.
The Associated Press contributed to this report.