Security experts are warning that Russian forces “cannot” reinforce their positions quickly enough and will now have no other option but “to dig in and wait for winter” and then fight on frozen frontline.

Ukraine have made “huge gains” and are expected to make more major inroads the occupied territories as their counteroffensive gathers pace.

Security and defence analyst Professor Michael Clarke said that Russia has dwindling troops and it is therefore impossible for them to consolidate positions and Russia now has a “problem” in deciding if they should defend Severodonetsk.

The momentum has now shifted to Ukraine’s advantage, and “it has fundamentally shifted” which is “a disastrous situation for Russia now.”

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The former CIA director has warned that there “will be more tough fighting” with “more punishing Russian strikes on civilian infrastructure” in Ukraine.

Professor Clarke told Sky News, “The Ukrainians now control the key road between Savatova and Kremenna and they look as if they might be trying to encircle the Russians from the north in Severodonetsk.

“That gives the Russians a problem.

“If they cannot, or will not, fight for Severodonetsk, then their next plausible defence line is probably some sort of major dog leg further to the east.

“Whatever they do, the Russians cannot reinforce their positions very quickly. Somewhere, they are going to have to dig in and wait for winter.”

Petraeus gave his thoughts on how the war could develop over time, he said, “There will be more tough fighting, more casualties, more punishing Russian strikes on civilian infrastructure – but Ukraine will over time, I think, retake the territory that Russia has seized since February 24.

“And it’s even conceivable now that they could retake Crimea and the Donbas.

“And, by the way, with what’s going on the frontline, there’s insurgence activity now picking up in the Russian-rear areas carried out by Ukrainians there as well.”

Petraeus added, “This is going to take time. There will be tough fighting – all of that.

“But this is a disastrous situation for Russia now.”

The former CIA director was asked if he thinks the momentum has now shifted to Ukraine’s advantage, Petraeus replied, “It has fundamentally shifted.

“And you know, I’m normally guarded and cautious about this.

“But the tide clearly has turned because the success of this offensive, as important as it is itself on the ground, what really is important is that it reflects a hugely important development, a new reality that Ukraine has been incomparably better than Russia in recruiting, training, equipping, organising and employing additional forces.

“While Russia has been struggling to do just that, literally running out of soldiers, ammunition, tanks, fighting vehicles and so forth, the Ukraine supported superbly by the US and NATO.

“Whereas Russia, even if it declared mobilisation today, could not reverse this fundamental reality.

“So, the implications are stark.”