Russian private army chief Wagner said Moscow is not providing her with the ammunition she needs to take control of Bakhmout.
Russian troops – from Wagner and Russian regular forces – attempt to take the eastern city from Ukraine.
But Wagner boss Yevgeny Prigozhin complained about the lack of ammunition, saying it could be “mere bureaucracy or betrayal”.
Relations between Wagner and Moscow seem increasingly strained.
The Wagner Group has tens of thousands of troops in Ukraine – some recruited directly from Russian prisons – and became a key part of the invasion of Moscow.
In the post on Sunday, Mr Prigozhin said the documents were signed on February 22, with the ammunition due to be sent to Bakhmut the following day.
But most have not been dispatched, he said, before suggesting it could be deliberate.
Separately, in a video released on Saturday – but apparently filmed in February – Mr Prigozhin said his men feared they would be “marked” as scapegoats if Russia lost its war in Ukraine.
“If we back down, then we will forever go down in history as those who took the main step to lose the war,” he said.
“And this is precisely the problem of this same hunger for shells [shortage of ammunition]. This is not my opinion, but that of ordinary fighters…”.
“What if they [the Russian authorities] wanted to trap us, and that’s why they don’t give us ammunition, they don’t give us weapons, and they don’t let us resupply our staff, including among those imprisoned?
In Saturday’s video, Mr Prigozhin said Russia’s frontline would crumble without its troops.
“If now the PMC Wagner [private military company] withdraws from Bakhmut, then the whole front will collapse, and today the PMC Wagner cements it.
“On the one hand, we are drawing the entire Ukrainian army to ourselves, crushing it and destroying it, and not giving it the opportunity to concentrate in other sectors of the front.
“On the other hand, we’re moving forward and the others [an apparent reference to the Russian military] are kind of having to catch up to save face.”
Last month Mr Prigozhin complained that Defense Minister Sergei Shoigu and Chief of General Staff Valery Gerasimov were withholding ammunition deliveries to his troops.
Ukrainian troops are likely carrying out a “limited combat withdrawal” in eastern Bakhmut, the Institute for the Study of War (ISW) said on Monday.
But he added that Ukraine “continued to inflict high casualties” on Russian forces.
According to the ISW, the Russian military relied on Wagner in the months-long effort to seize Bakhmut and has since “reinforced Wagner’s forces in Bakhmut with Russian airborne elements and mobilized personnel”.
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 have not yet taken control.
“They don’t aim to save the city…their only aim is to kill people and carry out genocide of the Ukrainian people,” Marchenko told the Today programme.
Meanwhile, Ukrainian military officials said leaders of Russia’s 155th Brigade fighting near the town of Vuhledar, south of Bakhmut, resisted attack orders after suffering heavy casualties.
The Russian Defense Ministry said its forces struck a command center of the Ukrainian Azov regiment in the southeastern Zaporizhzhia region.