Ukrainian President Volodymyr Zelensky has urged Russian troops to flee for their lives after his forces launched an offensive to retake southern Ukraine, but Moscow says it halted the attack and inflicted heavy losses on Kyiv.

Ukraine said on Monday its ground forces had gone on the offensive in the south for the first time after striking Russian supply lines, in particular bridges across the strategically-important River Dnipro, and ammunition dumps.

In a late-night address on Monday, Mr Zelensky called on Russian forces to go home or be chased home.

‘‘If they want to survive – it’s time for the Russian military to run away. Go home,’’ he said.

‘‘Ukraine is taking back its own (land),’’ Mr Zelensky said, adding that he would not disclose Kyiv’s precise battle plans, but that his armed forces were doing their job.

The new offensive comes after several weeks of relative stalemate in a war that has killed thousands, displaced millions, destroyed cities and fuelled a global energy and food crisis amid unprecedented Western economic sanctions on Russia.

Russia captured swathes of Ukraine’s south near the Black Sea coast in its early phase, including in the Kherson region which lies north of the Russian-annexed Crimean peninsula.

Ukraine, now armed with sophisticated Western-supplied weapons, sees retaking the region as crucial to prevent Russian attempts to seize more territory further to the west that could eventually cut off its access to the Black Sea.

Oleksiy Arestovych, a senior Ukrainian presidential adviser, said Russian defences in the Kherson region had been ‘‘broken through in a few hours’’.

It was unclear which line of Russian defence, of which there are many, he was referring to.

Mr Arestovych also said Ukrainian forces were shelling ferries that Russia was using to supply a pocket of territory on the west bank of the Dnipro River in the Kherson region.

Britain, a close ally of Ukraine, said on Tuesday that Kyiv had stepped up its artillery barrage across the entire southern front, but said it was not yet possible to confirm the extent of Ukrainian territorial advances.

Vitaly Kim, governor of the Mykolaiv region, told Ukrainian TV: ‘‘Heavy fighting is going on. Our military is working around the clock. Liberation of the Kherson region is coming soon.’’

Unverified reports, images and footage on social media suggested that Ukrainian forces may have taken back some villages and destroyed some Russian targets in the south.

However, Russia’s defence ministry said the Ukrainian offensive had been halted in its tracks.

It said in a statement that Ukrainian forces had attempted to go on the offensive in three different directions in the southern Mykolaiv and Kherson regions, but had lost some 560 military personnel, 26 tanks and two warplanes.

‘‘Another attempt by the enemy to go on the offensive has fallen apart,’’ it said.

Heavy Russian shelling of Kharkiv, Ukraine’s second city, was reported.

At least five people were killed and seven were wounded, the mayor of Kharkiv, Ihor Terekhov, wrote on the Telegram messaging app.

The Zaporizhzhia nuclear power plant in central southern Ukraine, captured by Russian troops in March but still manned by Ukrainian staff, has been a hotspot in the conflict, with both sides trading blame for shelling in the vicinity.

Russian-installed authorities accused Ukrainian troops of firing two shells that exploded near a spent fuel storage building at the plant, the TASS news agency reported.

A mission from the International Atomic Energy Agency (IAEA) is headed to the nuclear plant, Europe’s largest, and is due later this week to inspect and assess any damage.

Led by IAEA chief Rafael Grossi, the mission will evaluate working conditions and check safety and security systems, the Vienna-based organisation said.

-Reuters