A surprise Ukrainian robot offensive in the eastern Donbas region appears to have reversed over a year of Russian territorial gains, according to security experts, dealing a strategic blow to Russian President Vladimir Putin's territorial ambitions.

Details of the Ukrainian counteroffensive, dubbed "Operation Vivaldi," have started to come to light in recent weeks, offering rare insight into the fast-changing nature of frontline combat operations amid Russia's full-scale invasion.

As part of the ongoing operation, which started in May, security experts said Ukraine's Third Army Corps had been able to deploy large bomber drones and remote-controlled robots behind enemy lines around Lyman in the northern Donetsk Oblast, apparently catching Russian forces off guard.

Russian forces have been seeking to encircle Ukraine's so-called fortress belt cities that make up the remaining Ukrainian-held territory in the region for the past 12 months.

The third phase of Operation Vivaldi, however, liberated an additional 51 square kilometers, taking the total area of territory retaken to 176 square kilometers, Ukrainian Brigadier-General Andrii Biletskyi said on Sept. 28. The latest phase of fighting inflicted over 2,000 Russian losses, with over 250 Russian soldiers taken prisoner, he added.

The experience of Operation 'Vivaldi' has demonstrated that the systematic replacement of human personnel with robots is changing the paradigm of modern warfare.Serhii KuzanChairman of the Ukrainian Security and Cooperation Center

Patrick Bolder, a defense expert and strategic advisor at the Hague Center for Strategic Studies (HCSS), described the offensive as a "big bloody nose" to Putin's minimal war aim of seizing Ukraine's Donbas region.

Alongside demonstrating an ability to keep details of the operation secret, running disinformation campaigns against Russia and orchestrating a months-long air operation to disrupt logistical supplies to the frontline, Bolder said Ukrainian forces had managed to secure control of the air in the Lyman area, denying Russia the use of its drones.

Ukrainian forces were then able to deploy their own drones to drop unmanned ground vehicles behind enemy lines to attack the Russian troops from behind, Bolder said, without endangering their own personnel.

"They put this together masterly," Bolder said.

For the Kremlin, "gaining the Donbas is very far away at the moment because the Ukrainians have come into better defendable positions now — which will make it harder for the Russians to try to do this again," he added.

A spokesperson for Russia's Embassy in London was not available to comment when contacted by CNBC on Friday.

'A powerful message to Moscow and Kyiv's partners'

Security experts made it clear that Ukraine's robot offensive should not be seen as a decisive breakthrough.

It does, however, send "a powerful message to Moscow and to Kyiv's partners that the highly innovative Ukrainian military remains capable of defeating Russia on the battlefield," according to David Kirichenko, an associate research fellow at the Henry Jackson Society, a U.K.-based national security think tank.

Ukrainian President Volodymyr Zelenskyy lauded the success of Ukraine's Third Army Corps when he met with soldiers carrying out the counteroffensive, describing the operation as "important in many ways" for Ukraine's defense overall, and the Donbas region especially.

"One particularly promising area now is replacing our warriors with ground robotic systems wherever possible," Zelenskyy said in a social media post on Sept. 30.

"During Vivaldi, this capability allowed us to accomplish many things. We will scale it up so that, more and more often, a drone rather than a warrior carries out the most dangerous tasks at the front," he added.

Russia has stepped up its attacks on Ukraine's capital and major cities in recent days, targeting data centers, communications and energy infrastructure as it seeks to disrupt the flow of information and force power cuts ahead of winter.

Ukrainian officials have expressed deep concern over the attacks, saying there is an urgent need for the country to be able to rapidly respond to missile and drone threats.

Ukraine, for its part, has sought to raise the cost of Russia's more than four-and-a-half-year war against it, targeting the country's oil refineries and logistics hubs.

'Changing the paradigm of modern warfare'

Robotic systems have become one of the key factors in Ukraine's counter-offensive operations, according to Serhii Kuzan, chairman of the Kyiv-based Ukrainian Security and Cooperation Center think tank.

"Their use has not only reduced the risks to personnel but has also changed the very logic of combat: where previously dozens or hundreds of soldiers were required, remotely controlled platforms are increasingly being deployed," Kuzan told CNBC by email.

"The experience of Operation 'Vivaldi' has demonstrated that the systematic replacement of human personnel with robots is changing the paradigm of modern warfare, enabling tasks to be carried out with minimal personnel losses."

Ultimately, Kuzan said Ukraine's counter-offensive in the Lyman area had been aimed at seizing the initiative on the battlefield, "which will ultimately force the Russian army to abandon the objectives set by its political leadership."

Ukrainian officials have reported that Russia's political leadership had previously set an objective to capture the Donetsk region by the end of the year, although the Russian military is said to now be seeking an extension to March next year.

"This is because, according to Ukrainian intelligence, on the main thrust directions of Kostiantynivka, Sloviansk and Kramatorsk, the Russian army is suffering its heaviest losses, accounting for 70–80 per cent of all casualties," Kuzan said.