KYIV, May 22 (Reuters) - Russia said on Monday it was battling a cross-border incursion by saboteurs who burst through the frontier from Ukraine, in what appeared to be one of the biggest attacks of its kind since the war began 15 months ago.

The governor of Russia's Belgorod region, Vyacheslav Gladkov, said a Ukrainian "sabotage group" had entered Russian territory in the Graivoron district bordering Ukraine and was being repelled.

But the Ukrainian outlet Hromadske cited Ukrainian military intelligence as saying two armed Russian opposition groups, the Liberty of Russia Legion and the Russian Volunteer Corps, both consisting of Russian citizens, were responsible for the attack.

A Ukrainian presidential adviser said on Twitter that the Kyiv government was watching the situation with interest but "has nothing to do with it".

The Russia Volunteer Corps published video footage late on Monday which showed what it said was a fighter inspecting a captured armoured vehicle. Another video showed what it said were fighters operating an armoured vehicle on a country road.

Other videos posted on Russian and Ukrainian social media channels showed pictures and video of what were described as captured Russian servicemen and their identity documents.

Reuters was not able to independently verify the situation.