The head of Russia’s Wagner private army says it is not getting the ammunition it needs from Moscow, as it seeks to gain control of Bakhmut.
The eastern city has seen months of intense fighting, as Wagner and regular Russian troops try to seize it.
But Wagner boss Yevgeny Prigozhin says his army’s lack of ammunition could be “ordinary bureaucracy or a betrayal”.
Ukraine’s president and military commanders have agreed to strengthen their defence of Bakhmut.
Russia has appeared determined to capture the city for months, but many analysts say it has become a symbolic prize in the war and has little strategic value.
An apparent rivalry between the mercenaries and the regular Russian army seems to have intensified in recent weeks, and this is not the first time Mr Prigozhin has accused the Russian defence ministry of withholding the ammunition it needs.
In a social media post on Sunday, Mr Prigozhin said documents had been signed on 22 February, with ammunition expected to be sent to Bakhmut the next day.
But most had not been shipped, he said, before suggesting it could be deliberate.
And in a further sign of the rift, on Monday Mr Prigozhin said his representative was unable to access the headquarters of Russia’s military command. It is unclear where the headquarters is located.
Mr Prigozhin said it came after he wrote to the chief of Russia’s “special military operation”, Valery Gerasimov, about the “urgent necessity to give us ammunition”.
Separately, in a video uploaded on Saturday – but seemingly filmed in February – Mr Prigozhin said his men feared that they were being “set up” as scapegoats in case Russia lost its war in Ukraine.
“If we step back, we will go down in history as the people who took the main step to lose the war,” he said.
“And this is precisely the problem with the shell hunger [ammunition shortage]. This is not my opinion, but that of ordinary fighters…
“What if they [the Russian authorities] want to set us up, saying that we are scoundrels – and that’s why they are not giving us ammunition, not giving us weapons, and not letting us replenish our personnel, including [recruiting] prisoners?”
In Saturday’s video, Mr Prigozhin also said Russia’s front line would collapse without his troops.
He suggested Wagner fighters were taking on the “entire Ukrainian army… destroying it” and depriving it of the chance to concentrate on other parts of the front.
Ukraine’s troops were probably conducting a “limited fighting withdrawal” in eastern Bakhmut, the Institute for the Study of War (ISW) said on Monday.
But it added Ukraine was “continuing to inflict high casualties” on Russian forces.
Ukrainian President Volodymyr Zelensky said his country’s military commanders supported continuing operations to defend and strengthen positions in Bakhmut.
In a statement, President Zelensky’s office said he had held a meeting with Valery Zaluzhny, the head of Ukraine’s armed forces, and Oleksandr Syrsky, commander of the country’s ground forces.
They spoke “in favour of continuing the defensive operation and further strengthening our positions in Bakhmut”.
On Saturday, the deputy mayor of Bakhmut told the BBC that there was street fighting between Russian and Ukrainian forces.
However Oleksandr Marchenko said Russian troops had not yet gained control.
“They have no goal to save the city… their only goal is killing people and the genocide of the Ukrainian people,” Mr Marchenko told the Today programme.
Separately, Moscow’s Mr Shoigu has visited the occupied city of Mariupol during a trip to eastern Ukraine – a year after his troops besieged the city.
The defence ministry said he was inspecting work carried out to “restore infrastructure in the Donbas” – words that are likely to grate in Ukraine, given Russia’s responsibility for the destruction.