The feud between Moscow and the Wagner Group of mercenaries has reached an "unprecedented level", the UK has said.
In its latest defence intelligence update, the Ministry of Defence says for the first time, the head of the Wagner Group has claimed the Russian army has employed "deliberate, lethal force" against his units.
The MOD added that following "an altercation", it is likely that the Wagner Group has detained a Russian army brigade commander.
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Most of Wagner's forces have now been withdrawn from Bakhmut, the focus of much intense fighting in recent months, the MOD added.
With Russia short of reserve forces, the extent to which Wagner responds to the Russian MOD will be "a key factor" in the war over the coming weeks.
It comes as UK defence intelligence has reported "a substantial increase in fighting" along numerous sectors of the front, including those which have been relatively quiet for several months.
Has Ukraine's counter-offensive already begun?
Russia claims to have foiled a major Ukrainian offensive in Donetsk, with a grainy video apparently the evidence.
The Russian defence ministry said the country's troops thwarted a Ukrainian offensive, killing hundreds of troops and destroying multiple armoured vehicles in the process.
Ukraine has refuted the claims, accusing Russia of spreading lies, while experts believe this claimed offensive is a sign that "something has begun".