Western officials have said the sustainability of Russia’s attacks on Ukraine is “challenging”, but described the impact on their munitions and morale as “remarkable”.

Officials said the capture of Lysychansk over the weekend meant Russia had made “genuine headway” against the objective it claimed was the rationale for the invasion – the supposed liberation of the Donbas.

But one official said it “remains highly uncertain whether Russia will secure the limits of Donetsk Oblast this year”.

They also said there has been “better cooperation” amongst Russian forces in the south after General of the Army Sergey Surovikin took over command of the southern group of forces.

One official said: “Russia has made some significant command changes in recent weeks.

“Notably General of the Army Sergey Surovikin has taken over command of the southern group of forces, which is overseeing the occupation of southern Ukraine and the advances on the Donbas from the south.

“He’s a controversial figure even by the standards of Russian general officers.

“It is unclear whether it’s his influence which has led to the recent successes around Lysychansk, but certainly there’s been better cooperation amongst groups of forces on the Russian side than we saw in the earlier phases of the war.”

An official said Russia’s tactical success in the Donbas region “does not mean that we have changed our position”.

They said: “Ultimately, the sustainability of the Russian offensive is challenging but the costs Russia endures for each advance remain remarkable and there are very serious issues over the stocks of Russian munitions and of morale.

“It remains highly uncertain whether Russia will secure the limits of Donetsk Oblast this year.”

The western official said long-range weapon systems are starting to make a “significant operational difference for Ukraine – as seen by Russia’s evacuation of Snake Island and by increasing debilitating strikes behind Russia’s lines in the Donbas – which are further weakening Russia’s existing vulnerabilities”.