Russian forces have had some success in the eastern frontline city of Bakhmut, Ukrainian military officials said, adding that their fighters were still holding on in a months-long battle in which both sides have suffered heavy casualties.

In southern Ukraine, the United Nations nuclear watchdog chief said there had been a significant build-up in the number of troops in the region of the Russian-occupied Zaporizhzhia power plant and it could no longer be protected.

The mining city of Bakhmut and surrounding towns in the eastern industrial region of Donetsk have been the focal point of assault for much of the 13-month-long invasion by Russia of neighbouring Ukraine.

"Enemy forces had a degree of success in their actions aimed at storming the city of Bakhmut," the General Staff of the Ukrainian armed forces said in a regular report late on Wednesday.

"Our defenders are holding the city and are repelling numerous enemy attacks."

The average number of daily Russian attacks on the front line reported by Ukraine's general staff has declined for four straight weeks since the beginning of March, to 69 in the past seven days from 124 in the week of March 1-7. Just 57 attacks were reported on Wednesday.

Reuters journalists near the front west of Bakhmut and further north also reported a notable decline in the intensity of Russian attacks last week.

Russian officials say their forces are still capturing ground in street-by-street fighting inside Bakhmut.

Reuters was not able to verify battlefield reports.

'CAN'T BE PROTECTED'
The Zaporizhzhia power station was captured by Russian troops in the opening weeks of the war a year ago and attempts to reduce fighting and shelling around it have failed despite fears of a nuclear disaster.

Rafael Grossi, head of the International Atomic Energy Agency (IAEA), on a repeat visit to the plant on Wednesday, told Russian reporters there had been a "significant increase" in the number of troops in the region.

"It is obvious that military activity is increasing in this whole region. So the plant can't be protected," he said.

A recording of the briefing was made available to Reuters.

Grossi said he was putting aside plans for a security zone around the plant so he could propose specific protection measures acceptable to both Russia and Ukraine.

The plant was a prized part of Ukraine's energy network and accounted for about 20% of national power generation before the invasion. It has not produced any electricity since September, when the last of its six reactors was taken offline.

The IAEA has had monitors stationed at the plant since September, when Grossi travelled to the facility as fears of a potential nuclear accident mounted.

Russian forces shelled towns in central Zaporizhzhia region, including the contested centre of Hulyaipole, the Ukrainian general staff said.

'SHIFTED FOCUS'
Ukrainian military analyst Oleh Zhdanov, who has served in the military, said that while the offensive on Bakhmut remained intense, "the conclusion is that Russian troops are beginning to rush about from place to place".

"It now appears that the enemy has shifted its focus to the city itself - that is where the heaviest fighting is now taking place," Zhdanov said in a YouTube video.

Another Ukrainian military analyst, Roman Svitan, who is also a colonel in the Ukrainian reserves, said the situation in Bakhmut has stabilised and the Ukrainians' main task there, to destroy Russian forces, was being fulfilled.

Ukraine's deputy defence minister, Hanna Malyar, said in a social media post that while losses were inevitable "the enemy's losses are many times greater".

The Ukrainian military also said there was renewed shelling of Kherson city in the south, along with other towns on the west bank of the Dnipro River that bisects the country.

The Ukrainian air force destroyed a Russian Su-24M bomber, it said. Rocket and artillery in the past 24 hours struck two areas of concentration of Russian forces, an ammunition depot and two fuel depots, it said.

What Russia has called a "special military operation" to reduce a threat to its own security has killed thousands of troops on both sides, tens of thousands of Ukrainian civilians and displaced millions. The invasion also shook the global economy and disrupted international relations.

Britain, the United States and European allies of Ukraine have provided it with weapons and money, describing the invasion as an imperial-style land grab by Russia.