Russia's Wagner military group has taken full control of the eastern part of the Ukrainian city of Bakhmut, its founder Yevgeny Prigozhin said on Wednesday.
If his claims are proven true, Russian forces would be in full control of almost half of the city.
“Units of the private military company Wagner have taken control of the eastern part of Bakhmut,” Mr Prigozhin said on the Telegram messaging platform.
“Everything east of the Bakhmutka River is completely under the control of Wagner,” he said.
Ukrainian President Volodymyr Zelenskyy has said his army remained intent on defending Bakhmut and warned the Russian army would have an “open road” into eastern Ukraine if it captured the town.
The battle for Bakhmut, a salt-mining town with a population of 80,000 before the fighting began, has lasted for months and become one of the war’s bloodiest battles.
Mr Zelenskyy said his armed forces were resolved to fight for Bakhmut.
“I had a meeting with the chief of staff yesterday and the chief military commanders online and offline … and they all talk that we have to stand strong in Bakhmut,” he said.
“Of course, we have to think about the lives of our military. But we have to do whatever we can whilst we're getting weapons, supplies and our army is getting ready for the counter-offensive.”
He added: “We understand that after Bakhmut they could go further. They could go to Kramatorsk, they could go to Sloviansk, it would be open road for the Russians after Bakhmut to other towns in Ukraine, in the Donetsk direction,” Prersident Zelenskyy said.
Last Friday, Mr Prigozhin said his forces had the city “practically surrounded”. The mercenary boss has appeared to issue premature claims of territorial gains in the past.
He has also clashed repeatedly with Russia's defence ministry, complaining that his men have been starved of the ammunition they need.
In a separate video posted on his Telegram channel later on Wednesday, standing in front of a tank with explosions raging in the background, Mr Prigozhin called on Mr Zelenskyy to “take the old people and children” out of the battered city.
Russian forces have been trying to seize Bakhmut for months.
If Russian forces do capture the city it will become their first significant territorial gain since last summer.
“Capturing [Bakhmut] will allow for further offensive operations deep into the defence lines of the Armed Forces of Ukraine,” Russian Defence Minister Sergei Shoigu has said.
Mr Prigozhin has estimated that between “12,000 and 20,000" Ukrainian troops were defending the city.
He said that while “very tough battles are ongoing both day and night”, Ukraine's fighters “are not running away”.
Both sides have said the Bakhmut battle has cost a significant number of troops, but neither gave figures.