Ukraine, which describes Russia's actions as an imperial-style war of conquest, said on Thursday its planes had struck five Russian strongholds around the city of Kherson and another city in the area.

British military intelligence, which helps Ukraine, said it was likely that Ukrainian forces had also established a bridgehead south of a river which runs along the wider Kherson region's northern border.

"Ukraine’s counter-offensive in Kherson is gathering momentum," it said in a statement.

British intelligence said the strategy was starting to isolate Russian forces in the Kherson region.

"Russia's 49th Army, stationed on the west bank of the Dnipro River, now looks highly vulnerable," it said in an intelligence bulletin.

Kherson city was now virtually cut off from the other territories occupied by Russia.

"Its loss would severely undermine Russia's attempts to paint the occupation as a success," it said.

A Ukrainian strike on Wednesday on the Antonivskyi bridge, the sole span serving Kherson city, prompted its closure to traffic. That forced Russia to open a ferry service, the route of which it said would constantly change for security reasons.

Ukraine says it has retaken some small settlements on the region's northern edge in recent weeks as it tries to push Russian forces back, a potential prelude to what Kyiv has billed as a major counter-offensive to retake the south.

Russia said on Thursday it was unfazed, with the defense ministry saying its planes had attacked a Ukrainian infantry brigade in the far north of the Kherson region and killed more than 130 of its soldiers in the last 24 hours.

Kirill Stremousov, the deputy head of the Russian-appointed military-civilian administration running the Kherson region, has also dismissed Western and Ukrainian assessments of the battlefield situation.

"(Ukrainian President Volodymyr) Zelenskiy's statement that Kherson region will be liberated in three to six weeks is...a lie. All these counter-offensives that result in a large number of Ukrainian casualties are coming to nothing," Stremousov told Russia's RIA news agency on Wednesday.

Russia continues to carry out its own daily strikes against targets across Ukraine.

Five people were killed and 25 wounded in a Russian missile strike on a flight school in the central Ukrainian city of Kropyvnytskyi on Thursday, the regional governor said.

Fierce fighting is also underway in eastern Ukraine where Russia is trying to take control of the entirety of the industrialized Donbas region.

Ukraine confirmed late on Wednesday that Russian forces had captured the Soviet-era coal-fired Vuhlehirsk power plant, Ukraine's second-largest, in what was Moscow's first significant gain in Donbas in more than three weeks. Kyiv played down the importance of the setback.