Ukraine on Friday announced that its forces were entering the city of Kherson, hailing an "important victory" after Russia said its troops had retreated from the only regional capital it had captured in nearly nine months of fighting.

Ukraine's parliament published pictures of people carrying Ukrainian flags in the centre of Kherson, the capital of the eponymous region and close to the Black Sea.

Hours earlier, Russian strikes killed seven people in Mykolaiv, a nearby city in southern Ukraine on the Black Sea that the Russians have failed to capture but subjected to months of attack.

"Kherson is returning to Ukrainian control and units of the Armed Forces of Ukraine are entering the city," Kyiv's defense ministry said on social media.

It added that its artillery teams had clear views over Russia's routes to retreat and warned: "Any attempts to oppose the Armed Forces of Ukraine will be stopped."

Kherson was the first major urban hub to fall after President Vladimir Putin ordered Russian troops into Ukraine on February 24.

Its full recapture by Kyiv would be a political and symbolic blow to Putin and open a gateway for Ukraine's forces to the entire Kherson region, with access to both the Black Sea in the west and Sea of Azov in the east.

"Ukraine is gaining another important victory right now and proves that whatever Russia says or does, Ukraine will win," Foreign Minister Dmytro Kuleba wrote on social media.

He posted an amateur video showing Ukrainians removing a billboard near Kherson that proclaimed: "Russia is here forever".

'In tears'

In Kyiv, the news was met with joy and suspicion.

"I was in tears. I was so happy our territory is being returned," said Sergiy, a 26-year-old, who works in IT and who declined to give a second name.

"Why did they leave? Because they came with no purpose and they realized they had to leave," he added.

Svetlana, a pensioner from Kherson, said it was not clear why Russia had retreated.

"Everyone will interpret it in their own way. They still have certain artillery and missiles ... maybe they retreated just to bomb Kherson," she told AFP.

The Russian defense ministry said earlier: "The transfer of Russian troops to the left [eastern] bank of the Dnipro River was completed. Not a single piece of military equipment and weapons was left on the right (western) bank".

While it would appear a major Russian setback, the Kremlin insisted that Kherson was still part of Russia and that it did not regret annexing the entire Kherson region at a lavish ceremony in late September.

"This is a subject of the Russian Federation. There are no changes in this and there cannot be changes," spokesman Dmitry Peskov told reporters.

A full Ukrainian recapture of the Kherson region would disrupt a vital land bridge for Russia between its mainland and the Crimean peninsula, which Moscow annexed from Ukraine in 2014.

'Cynical' attack

Ukrainian officials were initially wary after Moscow announced this week that it would pull forces to defensive positions on the east bank of the river in Kherson.

Kherson was one of four regions of Ukraine that Putin claimed to have annexed during the September ceremony, vowing at the time to use all available methods to defend it.

Asked by reporters whether Russia regretted annexing Kherson, Peskov said the Kremlin had "no regrets" about the move.

Earlier on Friday, a Russian strike on a residential building in Mykolaiv killed seven people, regional governor Vitaliy Kim said on social media.

An AFP journalist at the scene saw a gaping hole gouged out of a Soviet-style residential building with emergency workers in yellow helmets on site clearing rubble.

Ukrainian President Volodymyr Zelensky branded the strike a "cynical response to our successes at the front".

He announced late Thursday that his forces had recaptured more than 40 towns and villages in southern Ukraine during a counter-offensive begun in August.

On Thursday, the United States announced a new $400-million security assistance package for Kyiv, including defence systems and surface-to-air missiles.

"(With) Russia's unrelenting and brutal air attacks on Ukrainian civilian and critical infrastructure, additional air defense capabilities are critical," Deputy Pentagon Press Secretary Sabrina Singh told journalists.

© Agence France-Presse