Russian forces in eastern Ukraine captured the centre of the railway hub town of Lyman and encircled most of Sievierodonetsk, Ukrainian officials acknowledged on Friday, as Kyiv’s forces fell back in the face of Moscow’s biggest advance for weeks.

Ukraine insisted its forces were still holding firm at new defensive lines in the eastern Donbas region, despite the apparent Russian advances on two of the major fronts there, battles that showed how momentum has shifted in recent days.

Moscow’s separatist proxies said they were fully in control of Lyman, a railway hub that Russia has been attacking from the north in one of the main axes of its advance.

Ukrainian officials acknowledged that Russia had captured most of the town. But the defence ministry said forces were still holding out in northeastern and southwestern districts, blocking the Russians from launching an advance towards Sloviansk, a major city a half-hour drive further southwest.Further east, Russian forces had encircled two-thirds of the city of Sievierodonetsk, Luhansk regional governor Serhiy Gaidai said.

The city is the easternmost major population centre still held by Ukraine in the Donbas, and Russia has been trying to trap Ukraine’s main fighting force inside it and its twin city Lysychansk across the Siverskiy Donets river.

Ninety per cent of buildings in Sievierodonetsk were destroyed, Gaidai said. Oleskiy Arestovych, an adviser to Ukraine’s President, said in an interview overnight that Lyman had fallen, and that the well-organised Russian attack there showed that Moscow’s military was improving its tactics and operations. After being driven back from Kyiv in March and from the outskirts of Kharkiv earlier this month, Russian forces are staging their strongest advance in weeks in the eastern Donbas region. The advance has gained ground since Russian forces broke through Ukrainian lines south of Sievierodonetsk.