“Russia seems to have been better prepared for this operation,” said John Hardie, deputy director of the Russia programme at the Foundation for Defence of Democracies.

“And this operation is considerably smaller in scale and its objectives,” he told The Telegraph. He described the force size as “modest”, likely totalling less than a battalion of 500 soldiers.

“At present, we are essentially talking about a limited number of Ukrainian units advancing through the fortified strip along the border, which the Russians have been actively reinforcing for two years,” said Olesia Horiainova, co-founder of the Ukrainian Security and Cooperation Centre, a Kyiv-based think-tank.

By breaching Russia’s defensive lines, Kyiv’s forces have improved their tactical position and seized back some of the initiative, “forcing Russia to react to the plans of the Ukrainian command rather than imposing its own,” said Ms Horiainova.

“Ukrainian command has repeatedly demonstrated its ability to surprise the world with effective and unexpected offensive operations, including on Russian territory.”

While the Kremlin has been defiant, there is dissent in Moscow.

“How did this happen? Russian forces completely f----d up... They just gave it up. They just left. After that, the enemy occupied it,” fumed well-known Russian war correspondent Vladimir Romano on Sunday as he reported the loss of Demidovka.

Reports suggest that some of Ukraine’s best units have been deployed to the operation as well as dozens of US-supplied Bradley fighting vehicles.

Rybar, an influential pro-Kremlin military blogger, said fierce battles were ongoing in Demidovka on Wednesday as Russian troops fought back. “The enemy has repeatedly tried to enter our strongholds across the border,” he wrote on Telegram.

“The situation remains tense,” he said, and cited Russian military reports that Ukraine was continuing to mass troops and vehicles on the border.