Russia had expected it could march across Ukraine without much of a fight when troops invaded for a “special operation” in February – but they have faced a valiant resistance and weeks of fighting.

Putin’s army chiefs have become increasingly frustrated with the scaled-down offensive in eastern Ukraine and his handling of the deadly war – with a vicious “blame game” exploding in Moscow.

Military bosses have now urged Putin to drop the term “special operation” used for the invasion and instead declare war on Ukraine – which would enable the mass mobilisation of Russian troops.

An all-out war would allow Moscow to draft in more conscripts, impose martial law and also bid for support from its international allies – such as Belarus.

A source close to Russian military chiefs told The Telegraph: “The military are outraged that the blitz on Kyiv failed.

“People in the army are seeking payback for failures of the past, and they want to go further in Ukraine.”

The UK’s Defence Secretary Ben Wallace warned Putin could use Russia’s victory day parade on May 9 to announce the mobilisation of his reserves for a final push in Ukraine.

He told LBC: “He is probably going to declare… that we are now at war with the world’s Nazis and we need to mass mobilise the Russian people.

“Putin, having failed in nearly all objectives, may seek to consolidate what he’s got.. and just be a sort of cancerous growth within the country.

“We have to help Ukrainians effectively get the limpet off the rock and keep the momentum pushing them back.”

And ex-Nato chief Richard Sherriff warned the West must “gear itself up” for a “worst case scenario” with Russia in Ukraine.

The editor of the state-owned broadcaster RT Margarita Simonyan said it was “more probable” that Putin would launch a nuclear strike than allow Russia to lose the war.

Mr Sherriff told BBC Radio 4: “The worst case is war with Russia. By gearing itself up for the worst case, it is most likely to deter Putin because ultimately Putin respects strength.”

Putin’s top generals are already said to be turning on the tyrant over the invasion as fears of a military coup mount.

The powerful Russian “siloviki” – or security bloc – have reportedly blamed Putin for a “serious error” after retreating from the capital of Kyiv and concentrating on taking control of the Donbas in the east.

Thousands more Russian troops have poured into eastern Ukraine – but they are making limited advances.

Putin’s military chief General Valery Gerasimov is now thought to be heading to the frontline to inject “momentum” and “authority” into the struggling troops.

“There are credible reports that he is going to go forward,” one western official reportedly said.

“In overall terms, it does show the continuing command and control challenges that the Russians are having.”