The head of Wagner mercenary force Yevgeny Prigozhin has warned that the Russian positions around the eastern Ukrainian city of Bakhmut were in peril unless his troops got ammunition.
Western media and analysts are treating it as the latest sign of tension between the Kremlin and the private militia chief.
The Ukrainian military also reported leaders of Russia's 155th Brigade fighting near the town of Vuhledar, south of Bakhmut, were resisting orders to attack after sustaining severe losses in attempts to capture it.
For its part, the Russian Defense Ministry on Sunday said Russian forces had hit a command center of the Ukrainian Azov Regiment in the southeastern Zaporizhzhia region. The ministry did not elaborate on the attack.
Reuters could not independently verify the battlefield accounts.
Wagner chief Prigozhin said Russia's front lines near Bakhmut could collapse if his forces did not receive the ammunition promised by Moscow in February.
"For now, we are trying to figure out the reason: is it just ordinary bureaucracy or a betrayal," Prigozhin, referring to the absence of ammunition, said in his press service Telegram channel Sunday.
The mercenary chief regularly criticizes Russia's defense chiefs and top generals. Last month, he accused Defense Minister Sergei Shoigu and others of "treason" for withholding supplies of munitions to his men.
In a nearly four-minute video published on the Wagner Orchestra Telegram channel Saturday, Prigozhin said his troops were worried that the government wanted to set them up as possible scapegoats if Russia lost the war.
"If Wagner retreats from Bakhmut now, the whole front will collapse," Prigozhin said. "The situation will not be sweet for all military formations protecting Russian interests."
A Russian victory in Bakhmut, with a prewar population of about 70,000, would give it the first major prize in a costly winter offensive, after it called up hundreds of thousands of reservists last year.
Russia says it would be a stepping stone to completing the capture of the Donbass industrial region, one of its most important objectives.
Volodymyr Nazarenko, a commander of Ukrainian troops in Bakhmut, said that there had been no order to retreat and "the Defense is holding" in grim conditions.
"The situation in Bakhmut and around it is very much hell-like, as it is on the entire eastern front," Nazarenko said in a video posted on Telegram.
Ukraine's military said early Monday its forces had repelled 95 Russian attacks in the Bakhmut area over the previous day.
"The situation in Bakhmut can be described as critical," Ukrainian military analyst Oleh Zhdanov said in a video commentary.
Russia's President Vladimir Putin ordered troops into Ukraine on a "special military operation" just over a year ago.
Since then, tens of thousands of people have been killed, millions have fled and cities have been reduced to rubble but Ukrainian forces, with the help of Western weapons, have limited Russian advances to the east and south.
To the north of Bakhmut, Russian troops advanced towards the town of Bilohorivka, just inside the Luhansk region, and shelled several settlements in the direction of Kupiansk and Lyman, the Ukrainian military said.
To the south, the Ukrainian military said Russian forces made preparations for an offensive in the Zaporizhzhia and Kherson regions, shelling dozens of towns and villages including the city of Kherson, causing civilian casualties.
A woman and two children were killed by Russian mortar bombs in a village in the Kherson region, the head of Ukraine's presidential office said.
Ukraine's air force spokesperson, Yuriy Ihnat, said 13 kamikaze drones had been shot down on Sunday night.
The governor of Russia's Belgorod region bordering Ukraine said one person was wounded by falling debris on Monday after Russian forces shot down three missiles near the town of Novy Oskol.
Belgorod borders Ukraine's Kharkiv region and has repeatedly come under fire since the beginning of Russia's invasion. Ukraine almost never publicly claims responsibility for attacks inside Russia.
Near Vuhledar, southwest of the Russian-occupied city of Donetsk, Ukraine said senior officers of Russia's 155th Brigade, which Kyiv says suffered heavy recent losses, were refusing to obey orders to attack.
"The leaders of the brigade and senior officers are refusing to proceed with a new senseless attack as demanded by their unskilled commanders – to storm well-defended Ukrainian positions with little protection or preparation," Ukraine's military said in a statement.
Military analyst Zhdanov said two "Cossack" Russian units known as Steppe and Tiger had expressed frustration with their commanders and refused to take part in any new offensive on the hilltop town.
Reuters could not immediately verify the reports.
Russian Defense Minister Shoigu is on a rare visit to his forces in Ukraine, awarding medals and meeting commanders on the weekend.
On Monday, he visited the eastern city of Mariupol, captured by Russian forces last year after a months-long siege.
