The Russian army on Tuesday vowed to capture the east Ukrainian town of Bakhmut, a symbolic prize in months of fierce combat, as a precursor for offensives deeper into Ukraine.

The intense fighting in the east raged on as Ukraine said it had identified a soldier filmed being shot dead in a video that sparked outrage on social media and as UN chief Antonio Guterres headed to Kyiv for talks. The battle for Bakhmut -- a salt-mining town with a pre-war population of 80,000 -- has been the longest and bloodiest in Russia's more than year-long invasion that has devastated swathes of Ukraine and displaced millions.

Russia has appeared intent to capture it at all costs, despite analysts saying it has little strategic value. "Capturing (Bakhmut) will allow for further offensive operations deep into the defence lines of the Armed Forces of Ukraine," Russian Defence Minister Sergei Shoigu told military officials during a televised meeting on Tuesday.

In Kyiv, Ukrainian President Volodymyr Zelensky said the army was intent on defending Bakhmut despite a rumoured retreat under pressure from Russian forces, who have sought to capture the city for months. Russia's mercenary group Wagner has spearheaded the attack on Bakhmut and its head Yevgeny Prigozhin, who is locked in a rift with Russia's military leadership, appeared to mock Shoigu saying he had "not seen him" near the battlefield.

Prigozhin estimated that between "12,000 and 20,000" Ukrainian troops were still defending the city. He said that while "very tough battles are ongoing both day and night", Ukraine's fighters "are not running away." Ukraine got a boost on Tuesday though, when its western neighbour and fervent ally Poland announced the sending of 10 Leopard tanks this week.

Both sides have said the Bakhmut battle has cost a significant number of troops, but neither gave figures. Outside the town, a Ukrainian soldier told AFP that Kyiv was losing control. "Bakhmut will fall," one exhausted soldier said Monday in the town of Chasiv Yar, 10 kilometres (six miles) west of the front line.

Some units had started to retreat in "small groups", he said. Ukrainian officials say around 4,000 civilians remain in the town, which has been virtually flattened. "Approximately 38 children, as far as we know, remain in Bakhmut," Deputy Prime Minister Iryna Vereshchuk told regional media on Tuesday.

Amid fears of Moscow-allied Belarus, Ukraine's northern neighbour, entering the conflict, Minsk accused Ukrainian secret services on Tuesday of being behind a partisan plot damaging a Russian plane in the country last month. Longtime leader Alexander Lukashenko said 20 people had been detained in connection with attacking the plane. Regime opponents said partisans damaged the jet at an airstrip near the capital Minsk last month.

Lukashenko identified the main culprit as a joint Russian and Ukrainian citizen and lashed out against Ukraine's Zelensky. Lukashenko, who allowed his Russian ally Vladimir Putin to use Belarusian territory as a launchpad for his Ukraine invasion a year ago, said the alleged culprit was a "terrorist." Ukraine also said Tuesday that it had identified a soldier filmed being shot dead in a video widely shared on social media that sparked outrage.

Killing video goes viral 

The footage shows what appears to be a detained Ukrainian combatant standing in a shallow trench, smoking, and being shot after saying "Glory to Ukraine". The phrase spoken by the alleged Ukrainian soldier has trended on social media. Senior officials in Kyiv blamed Russian forces and called for justice. The Ukrainian army identified him as Tymofiy Shadura, a serviceman of the 30th separate mechanised brigade.

He had been missing since February 3 amid fighting near Bakhmut and his identity would be confirmed when his remains were returned, the military added. AFP could not independently verify where or when the footage was filmed or whether it showed -- as Ukrainian officials and social media users suggested -- a Ukrainian prisoner of war. On Monday, Zelensky said the video showed Russian forces "brutally killing" a Ukrainian serviceman. "We will find the murderers," he vowed. Guterres, the UN secretary-general, was travelling to Ukraine to meet Zelensky in his third trip since Russia's invasion, a UN spokesman said.