A Russian military convoy moves on a highway in an area controlled by Russian-backed separatist. AP

Ukrainian shelling on Saturday forced Russian troops to suspend the evacuation of people from a chemical plant in Sievierodonetsk, just hours after Moscow's forces took the city, Tass news agency quoted local police as saying.

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The fall of Sievierodonetsk, following weeks of some of the war's bloodiest fighting, is the biggest defeat for Ukraine since it lost control of the southern port of Mariupol in May.

Ukraine called its retreat from the city a "tactical withdrawal" to fight from higher ground in Lysychansk on the opposite bank of the Siverskyi Donets river. Pro-Russian separatists said Moscow's forces were now attacking Lysychansk.

A woman stands above Nadiia Chuhai lying in her bed inside Azot chemical plant's bomb shelter. Reuters

The fall of Sievierodonetsk — once home to more than 100,000 people but now a wasteland — transforms the battlefield in the east after weeks in which Moscow's huge advantage in firepower had yielded only slow gains.

Russia will now seek to press on and seize more ground on the opposite bank, while Ukraine will hope that the price Moscow paid to capture the ruins of the small city will leave Russia's forces vulnerable to counterattack.

President Volodymyr Zelenskiy vowed in a video address that Ukraine would win back the cities it lost, including Sievierodonetsk. But acknowledging the war's emotional toll, he said: "We don't have a sense of how long it will last, how many more blows, losses and efforts will be needed before we see victory is on the horizon."

Russian and pro-Russian forces had entered Lysychansk across the river.

Kyrylo Budanov, Ukraine's military intelligence chief, told Reuters that Ukraine was carrying out "a tactical regrouping" by pulling its forces out of Sievierodonetsk.

"Russia is using the tactic ... it used in Mariupol: wiping the city from the face of the earth," he said. "Given the conditions, holding the defence in the ruins and open fields is no longer possible. So the Ukrainian forces are leaving for higher ground to continue the defence operations."

Russia's defence ministry said "as a result of successful offensive operations" Russian forces had established full control over Sievierodonetsk and the nearby town of Borivske.

Oleksiy Arestovych, senior adviser to Zelensky, said some Ukrainian special forces were still in Sievierodonetsk directing artillery fire against the Russians. But he made no mention of those forces putting up any direct resistance.

Russia's Interfax news agency cited a representative of pro-Russian separatist fighters saying Russian and pro-Russian forces had entered Lysychansk across the river and were fighting in urban areas there.

Reuters