Ukrainian troops move by tanks on a road of the eastern Ukrainian region of Donbas. AFP

Ukraine officials said there was very little left to defend in the bombed-out eastern city, where hundreds of civilians remain trapped in a chemical plant.

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The order to withdraw on Friday came four months to the day since Russian President Vladimir Putin sent tens of thousands of troops over the border, unleashing a conflict that has killed thousands, uprooted millions and reduced whole cities to rubble.

Luhansk regional governor Serhiy Gaidai said troops in Sievierodonetsk had already received the order to move to new positions.

Russian President Vladimir Putin attends a meeting. File photo

"Remaining in positions smashed to pieces over many months just for the sake of staying there does not make sense," Gaidai said on Ukrainian television.

The withdrawal from Sievierodonetsk would mark the biggest reversal for Ukraine since the loss of the southern port of Mariupol in May.

The latest Russian advances appeared to bring the Kremlin closer to taking full control of Luhansk, one of Moscow's stated war objectives, and set the stage for Sievierodonetsk's twin city of Lysychansk to become the next main focus of fighting.

Vitaly Kiselev, an official in the Interior Ministry of the separatist Luhansk People's Republic — recognised only by Russia — told Russia's TASS news agency that it would take another week and a half to secure full control of Lysychansk.

Smoke rises after a military strike on a compound of Sievierodonetsk's Azot Chemical Plant. Reuters

Since then Moscow and its proxies have focused on the south and Donbas, an eastern territory made up of Luhansk and its neighbour Donetsk, deploying overwhelming artillery in some of the heaviest ground fighting in Europe since World War Two.

Ukraine on Friday again pressed for more arms, with its top general, Valeriy Zaluzhniy, telling his US counterpart in a phone call that Kyiv needed "fire parity" with Moscow to stabilise the situation in Luhansk.

Reuters