The chief of the Russian paramilitary group Wagner has said his fighters have "practically encircled" Bakhmut, the eastern Ukraine city the Kremlin has been trying to seize for months.
Only one road remains under Ukrainian control, Yevgeny Prigozhin added in a video posted online in which he called on the Ukrainian president, Volodymyr Zelenskiy, to abandon the city.
While the claims by the head of the mercenary group could not be confirmed, the situation in the embattled city appeared to be extremely precarious, amid evidence Ukraine was preparing extensive new defensive positions, including around the nearby city of Kramatorsk.
Large amounts of military traffic were seen on nearby roads on Friday, and there were claims that several bridges near the city had been destroyed. Video posted online showed the blowing up of a railway bridge over the Bakhmutka River to the east of the city, while other footage purported to show damage to a small road bridge.
Russian shelling was seen hitting routes leading west out of Bakhmut, in what appears to be an attempt to block Ukrainian forces' access in and out of the city.
Amid mounting speculation that Ukraine's defenders might be preparing to withdraw, Oleksiy Danilov, the secretary of the national security and defence council of Ukraine, said that in the battles for Bakhmut its defenders had inflicted casualties on the Russian attackers at a ratio of 7:1.
"It is difficult there, but keep in mind that every day our boys and girls send hundreds of them to where they are supposed to go, given that they came here to kill us," Danilov said.
Volodymyr Nazarenko, a deputy commander in the National Guard of Ukraine, told Ukrainian NV Radio the situation was "critical", with fighting going on "round the clock".
The commander of a Ukrainian drone unit active in Bakhmut, Robert Brovdi, who goes by the name "Madyar", said in a video posted on social media that his unit had been ordered by the military to withdraw immediately. He said he had been fighting there for 110 days.
On a visit to the western city of Lviv to see wounded soldiers recuperating in hospital, Zelenskiy thanked those who had been injured during the "heroic defence" of Bakhmut.
Statements on social media issued by Ukrainian soldiers fighting in the Bakhmut area appeared to suggest they were holding their positions, although under intense pressure.
"Friends, at the moment there is no withdrawal of troops from Bakhmut," said one soldier from the 93rd mechanised brigade on Twitter. "Fighting continues in all areas of the city, the 93rd stands and continues to perform the tasks assigned to it. The situation is actually complicated, but trust only official sources and do not panic."
In the video posted by Prigozhin, recorded on a building's rooftop four miles north of Bakhmut, the Wagner chief claimed his forces were increasingly fighting against old men and children rather than the professional Ukrainian army.
The footage showed what looked like three captured Ukrainians - an older man and two young boys - who looked frightened and asked to be allowed to go home. They appeared to be talking in what looked like a choreographed appearance under extreme stress.
On Thursday, Prigozhin had released another video showing his fighters inside Bakhmut. Reuters geolocated the footage to the east of Bakhmut, about 1.2 miles (2km) from the city centre.
Ukraine has vowed to defend "fortress Bakhmut" but it has faced Russian troops determined to seize the city, whose symbolic importance now outstrips its military significance.
Ukrainian officials said the fighting was becoming increasingly difficult, after Russia claimed several villages near Bakhmut in recent weeks.
The latest fighting came as the German chancellor, Olaf Scholz, flew to Washington to meet President Joe Biden in Washington later on Friday. Also on Friday, the US indicated it was set to announce additional military aid for Ukraine in a $400m arms package expected to include ammunition for existing systems.
