(AFP) - Ukrainian President Volodomyr Zelensky on Sunday paid tribute to his soldiers fighting in the "painful and difficult" battle for the country's frontline eastern Donbas region.

He was speaking after Ukraine's general staff reported that its forces had fought off "more than 130 enemy attacks" the previous day, including in Kupiansk, Lyman, Bakhmut and Avdiivka.

"The enemy continues its attempts to encircle the town of Bakhmut," it said early on Sunday, of the eastern city that Moscow has been trying to capture for months.

Ukraine has vowed to defend "fortress Bakhmut" which Russian troops seem determined to take. Analysts say the city, which has been virtually destroyed in the fighting, has little real strategic value.

But, as what has become the longest and bloodiest battle of the conflict drags on, its fate has acquired a symbolic importance, surpassing its military significance.

"I would like to pay special tribute to the bravery, strength and resilience of the soldiers fighting in the Donbas," Zelensky said in his daily address.

"This is one of the hardest battles. Painful and difficult."

The Donbas is made up of Donetsk and Lugansk, which Russia claims to have annexed despite never fully having controlled it.

Ukraine's troops, said Zelensky, had "repelled assaults, destroyed the occupier, undermined enemy positions and logistics, and protected our borders and cities".

But on Saturday, the US-based Institute for the Study of War (ISW) warned that Ukrainian supply routes to Bakhmut were narrowing.

"The Russians may have intended to encircle Ukrainian forces in Bakhmut, but the Ukrainian command has signalled that it will likely withdraw rather than risk an encirclement," it added.

- Russian rivalries -

Pro-Russian separatists in the Donetsk region posted a video purporting to show Wagner fighters in the suburbs north of Bakhmut, having taken control of the Stupki railway station.

Wagner, a private army headed by Yevgeny Prigozhin, has taken centre stage in the fight for the city, which has exposed rivalries with Russia's conventional forces.

Already on Friday, Prigozhin said his fighters had "practically encircled" Bakhmut and only one road remained under Ukrainian control.

Prigozhin, who has for weeks been publicising the advances of his men towards the eastern city, on Saturday posted on social media, standing to what he said were coffins containing bodies of Ukrainian soldiers being shipped to territory controlled by Kyiv.

Prigozhin regularly posts videos of himself alongside mercenaries, on the ground or even in a fighter jet, in contrast with Russian generals criticised for shirking the front line.

In a rare exception, Russia on Saturday released a video of Defence Minister Sergei Shoigu inspecting troops in frontline regions in Ukraine.

The ministry said Shoigu inspected an advance command post in the South Donetsk direction without specifying exactly where or when.

He was seen travelling in a helicopter and talking to a soldier in front of damaged buildings.

The ISW think tank said Shoigu went there "likely to assess the extent of Russian losses around Vugledar and the possibility of a further offensive in this direction".

- Zaporizhzhia 'hostage' -

While the epicentre of the fighting is in the east, the death toll from a strike this week on an apartment block in southern Zaporizhzhia rose to 13, Ukraine's state emergency service said. Among the dead was a small child, the agency added.

The Ukrainian presidency also said Russian shelling had killed five people on Saturday.

Zaporizhzhia is one of the four regions -- along with Donetsk, Lugansk and Kherson -- that Russia claims to have annexed but has never fully controlled.

But Moscow's forces have held the Zaporizhzhia nuclear power plant since March 4, 2022.

The plant has repeatedly made headlines and revived fears of nuclear catastrophes similar to the deadly Chernobyl disaster that shook Ukraine in 1986.

The exiled mayor of Energodar, which houses the station, told AFP that Russia uses the plant as a "nuclear shield" for its troops and equipment.

After Kyiv and Moscow blamed each other for shelling around the plant, the International Atomic Energy Agency (IAEA) posted observers there.

Zelensky said on Saturday that Russia had taken the nuclear power plant "hostage" a year ago and "turned the territory of the (power plant) into a de facto military training ground".

© Agence France-Presse