An armed group has crossed from Ukraine into Russia's Belgorod region and clashes there have injured a number of people, Russian authorities say.

Local governor Vyacheslav Gladkov said Russian forces were searching for "saboteurs", who he said had attacked Grayvoronsky district by the border.

Vladimir Putin's spokesman said the Russian president had been informed.

Ukraine denies responsibility and said Russian citizens from two paramilitary groups were behind the incursion.

Mr Gladkov said eight people had been hurt, including two people admitted to hospital after a village was shelled and three people who were hit by shrapnel in the town of Grayvoron.

Fighting had also damaged three houses and a local administrative building, and the situation remains "extremely tense", he said.

The governor said a "counter-terrorist operation" had been launched in the region, giving special powers to the authorities including on identity checks and communications surveillance.

BBC Verify has been analysing footage from the Belgorod region that emerged on social media on Monday.

So far, the team has located a video apparently filmed from a drone that features several armoured vehicles near a border checkpoint south of Belgorod. Additionally, BBC Verify has geolocated footage of helicopters operating in the region.

The footage is recent, but it is hard to say for certain from the videos what the exact sequence of events is.

Kyiv said those behind the ongoing incident were from groups called the Liberty of Russia Legion and the Russian Volunteer Corps (RVC).

The Liberty of Russia Legion - a Ukraine-based Russian militia which says it is working inside Russia to overthrow President Putin - said on Twitter on Monday it had "completely liberated" the border town of Kozinka. It said forward units had reached the town of Grayvoron, further east.

However Mr Putin's spokesman Dmitry Peskov told Russian news agencies that efforts were underway to eliminate the sabotage group, and said its purpose was to draw attention away from the eastern Ukrainian town of Bakhmut - which a Russian mercenary group claims to have taken control of after months of intense and bloody fighting.

"We perfectly understand the purpose of such sabotage - to divert attention from the Bakhmut direction, to minimise the political effect of the loss of Artemovsk [Bakhmut] by the Ukrainian side," he said.

Ukrainian presidential advisor Mikhaylo Podolyak said his country was watching events in Belgorod "with interest", but "has nothing to do with it".

"As you know, tanks are sold at any Russian military store, and underground guerrilla groups are composed of Russian citizens," he added.

Ukraine has previously denied responsibility for reported sabotage attacks on Russian territory.

The latest incident comes ahead of a widely expected counter offensive by Kyiv against invading Russian forces.

In April, Russia accidentally dropped a bomb on the city of Belgorod, which lies 40 km (25 miles) north of the border with Ukraine.

More than 3,000 people were evacuated from their homes after an undetonated explosive was found days later. (BBC)

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