The Russian defence ministry said General Sergey Surovikin had been appointed "commander of the Joint Grouping of Forces in the areas of the special military operation", using the Kremlin's term for the offensive.

The decision was announced after Moscow's forces were pushed back by Kyiv in recent weeks in areas the Kremlin had declared Russian "forever".

According to the ministry's website, Surovikin is 55, born in Siberia's Novosibirsk.

He has combat experience in the 1990s conflicts in Tajikistan and Chechnya and, more recently, in Syria, where Moscow intervened in 2015 on the side of Bashar al-Assad's regime.

Until now Surovikin led the "South" forces in Ukraine, according to a defence ministry report in July.

The name of his predecessor has never been officially revealed, but some Russian media said it was General Alexander Dvornikov -- also a general of the Second Chechen War and Russian commander in Syria.

The decision -- unusually made public by Moscow -- comes after a series of crushing defeats suffered by the Russian army in Ukraine.

Russian forces were driven out of much of the northeastern Kharkiv region in early September by a Ukrainian counter-offensive that allowed Kyiv to retake thousands of square kilometres of territory.

Russian troops also lost territory in the southern Kherson region as well as the Lyman transport hub in eastern Ukraine.

The setbacks led to growing criticism of the military leadership, including from the elite.

Chechen leader Ramzan Kadyrov called for the firing of a top general last week, while a senior lawmaker -- Andrei Kartapolov -- urged military officials to stop "lying" about the situation on the battlefield.