The 41-year-old had been part of what Russia's official state news agency TASS had described as a "special military operation" when he died.

Further details about the circumstances of his death have not yet been revealed.

Uvitsky was secretary general of the Russian Kyokushin Federation, a karate organisation.

"Sergey has always been distinguished by his indomitable will on the way to the goal and never gave in to difficulties," a bulletin issued by the Federation said. 

"Sergey was a man with an incredibly strong spirit, a true patriot, devoted to his country until his last breath.

"Therefore, having completed a glorious sports career and becoming one of the leaders of the Russian Kyokushin Federation. 

"He stood up to defend the fatherland and died as befits a hero and a warrior while fulfilling a war mission."

Uvitsky had been a master of kyokushin karate and had won silver in the over-90 kilograms category at the 2010 European Championships.

After his competitive career was over he had become an administrator with the sport's federation.

When the war began, Uvitsky had joined the volunteer detachment of the Russian Union of Martial Arts Soyuz (RSMA).

Russian media agency Novosti reported that some 100 members of this unit had been given training by RSMA chairman Yuri Trutnev before embarking on the operation in Ukraine.

In Russia men between the ages of 18 and 27 have been subject to compulsory military call-ups during the war in Ukraine.

Russian Sports Minister Oleg Matytsin had insisted there should be no exemptions and the conscription has also received the backing of Russian Olympic Committee President Stanislav Pozdnyakov.