Russia will get “harder, tougher and more indiscriminate” in its attack on Ukraine as Vladimir Putin’s invasion effort lags “considerably” behind schedule, Defence Secretary Ben Wallace has warned.

Speaking to Sky News, Mr Wallace said the Russian forces in Ukraine were “moving very slowly on its objectives” and suffering “significant casualties” as they continue their assault on major cities.

“While we see some footage today of Kharkiv of Russian forces potentially in the middle of the city, they don’t hold the city,” the defence secretary said.

“It’s a city of 1.4m people and they are taking casualties, the Russians, and they are finding it very slow going.”

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Mr Wallace said the initial tactics of Russian forces “weren’t correct” and suggested they had thought they would conquer Ukrainian cities “in a few hours, rather than six days or longer”.

He added that Russia had been “surprised” by the strength and “clever plan” of Ukraine’s resistance, with reports that Ukrainians are using drones to attack Russian petrol and supply lines, as well as blowing up railway lines.

But the defence secretary warned, as the costs of the invasion rack up for Mr Putin, the Russian president was likely to resort to more brutal military tactics.

“I think the Russians have realised that the Ukrainians, by fighting, are costing them lots and lots of casualties and the Russians have taken significant casualties – far more than they would have planned,” he added.

“What they are therefore doing is trying to switch tactics and what you are seeing now are those heavy bombardments at night.

“They won’t come into the cities as much – they will, I’m afraid as we’ve seen tragically, carpet bomb cities indiscriminately in some cases.

“They will fly their air at night rather than daytime, because what we’ve seen is they get shot down in the daytime.

“And they will slowly but surely try and surround the cities and then either bypass them or bombard them.

“That is the brutality that I’m afraid we are witnessing and it’s going to get worse. I’ve warned before the Russian doctrine is to get harder and tougher and more indiscriminate.”