It was not immediately clear whether Ukrainian forces that had the Azot chemical plant, their last bastion within the city, had begun to march.
Yuri Butusov, a Ukrainian journalist who reports from the city, said the evacuation took place overnight before it was announced, and called the decision “bitter but delayed.”
“We left Severodonetsk,” he wrote on Facebook. “Tonight, Ukrainian units have left part of the Severodonetsk industrial zone in an organized manner. In the section of the front where I was, the retreat was attacked, but without losses.”
“Ukrainian soldiers fought with dignity for the city and did more than possible in such difficult conditions. Without bridges and without pontoons, every day of battle was a continuous heroism,” he added.
One month’s struggle
Severodonetsk has been the Ukrainian administrative capital of the Luhansk region since Russian-led separatists seized Luhansk in 2014.
Russia launched an offensive to capture it in April after Russian President Vladimir Putin ordered its generals to concentrate on capturing the Donbas after the failure of its assault on Kyiv.
They needed weeks of intensive combat preceded by massively destructive artillery bombardments to reach the outskirts of the city, despite deploying overwhelming concentrations of troops and equipment.
Street-to-street fights in the same city have lasted a month, with some areas changing hands several times until the Ukrainians were embedded in the Azot plant.
They had to be supplied with rafts after the Russians destroyed the bridges connecting the city with Lysychansk, controlled by Ukraine.