The deputy commander of Russia's military operations in Ukraine has urged fighters of the private military firm Wagner Group to obey President Vladimir Putin and not to join an armed rebellion called for by their leader.

Sergei Surovikin posted a message on social media on Saturday, urging Wagner fighters to return to their bases.
He is said to be close to Wagner leader Yevgeny Prigozhin.

The call follows the launch on Friday of a criminal investigation by Russia's Federal Security Service into Prigozhin for allegedly calling for an armed rebellion. The investigation was announced by Russia's prosecutor general.

Prigozhin has been increasingly at odds with the Russian defense ministry over operations in Ukraine.

On Friday, he said on social media that his forces were attacked by the Russian military.

Prigozhin even contradicted the Kremlin's justification of invading Ukraine.

Early on Saturday, Prigozhin said that Wagner fighters had crossed the border into the southern Russian region of Rostov. He warned that they will destroy anyone who stands in their way.

Tensions are rising in Rostov, where security is being enhanced in its main city.

Another Russian military official condemned Prigozhin's move as a coup in a video message.

Kremlin spokesperson Dmitry Peskov told Russian media on Saturday that Russian Prosecutor-General Igor Krasnov had informed Putin of the criminal case.

Ukraine's defense intelligence chief Kyrylo Budanov said in a social media post on Friday that Putin's fragile dictatorship had fallen. He said Russian totalitarians "began to gnaw at each other for power and money."

A US National Security Council spokesperson said Washington is closely monitoring the situation and is set to consult on the matter with allies and others.