It is unclear whether Russian frontline forces have “sufficient reserves or adequate morale” to withstand another concerted assault in eastern parts of Ukraine, according to defence experts.

British defence intelligence analysts believe Moscow has established a defensive line between the Oskil river and the town of Svatove following a successful counteroffensive from the Ukrainians, forcing the Kremlin to concede swathes of territory.

The zone is viewed as important partly because it sits along the border of the Luhansk region, part of the Donbas, which Russia has sought to “liberate” as one of its key war aims.

Any substantial loss of territory here would “unambiguously undermine” Vladimir Putin’s strategy for the conflict, the experts say.

In its latest update on the situation in Ukraine, the Ministry of Defence (MoD) said there was uncertainty over Russia’s capacity to withstand a further Ukrainian assault in the east of the country.

It comes after bodies were unearthed at a mass burial site near the city of Izium, in the Kharkiv region, which was recently recaptured from Moscow.

Ukrainian president Volodymyr Zelensky said the dead included children and adults, civilians and military.

“Tortured, shot, killed by shelling,” Mr Zelensky said.

“Even entire families are buried there: mother, father and daughter.”

Ukrainian MP Lesia Vasylenko said the recapture of Izium was a “huge strategic gain”.

“Ukraine is making confident advances in the northeast and the southeast of the country as well,” she told Sky News.

“We are making progress for our sake, but also for the sake of bringing back long-standing peace to the continent.”

Asked what she thought Mr Putin might do next, Ms Vasylenko said: “Nobody can be in the mind of that power-crazed leader.”

She added: “Whatever it is, we have to be prepared for it – we have to be prepared for it as Ukrainians, [and] you have to be prepared for it in the UK.

“And actually, people worldwide need to be prepared that something absolutely atrocious, another crazy move, can happen any time.

“For that, Ukrainians need to have the weapons to protect themselves, the ammunition in the necessary amount to protect themselves.

“And the West needs to be prepared to apply every sanction that there is in the book against Russia.”