Ukraine says it has arrested fugitive pro-Russian politician Viktor Medvedchuk, seen as President Vladimir Putin's closest ally in the country.

Ukraine's security service SBU posted a photo purportedly showing Mr Medvedchuk in handcuffs and wearing Ukrainian military fatigues.

He had been under house arrest in the capital Kyiv on suspicion of treason - but escaped soon after Russia invaded Ukraine on 24 February.

Mr Medvedchuk, 67, denies wrongdoing.

In his nightly video address to the nation on Tuesday, Ukrainian President Volodymyr Zelensky offered to exchange Mr Medvedchuk for Ukrainian "boys and girls who are now in Russian captivity".

He earlier wrote on Facebook that Mr Medvedchuk had been arrested after a "special operation" by the SBU.

The security service said in a statement: "You can be a pro-Russian politician and work for the aggressor state for years. You may have been hiding from justice lately. You can even wear a Ukrainian military uniform for camouflage.

"But will it help you escape punishment? Not at all! Shackles are waiting for you and same goes for traitors to Ukraine like you."

Mr Medvedchuk, whose daughter has President Putin as a godfather, is a wealthy businessman who leads the pro-Russian Opposition Platform - For Life party.

Russia on Tuesday said it was checking information about the reported arrest.

Mr Putin's spokesman Dmitry Peskov said this was because "so many fakes are now coming from Ukraine" that "now everything needs to be checked".

A wealthy businessman with close ties to the Russian leader, for years he was tolerated in Ukrainian political circles because he was seen as an important channel of communication with the Kremlin. He also played an important role as a go-between with Russian-backed separatists who seized areas of eastern Ukraine.

He has regularly been photographed alongside Vladimir Putin - while watching a Formula One race in Sochi, at martial arts meetings and hosting the Russian leader at his villa in Crimea.

Russia's president said in 2019 that they met from time to time. "I would not say that we are very close but we know each other well," he told filmmaker Oliver Stone.

Mr Medvedchuk has repeatedly described the accusations against him as "political repression".

He was charged with treason in May 2021 and placed under house arrest. Months later he was also accused of trying to buy coal from mines in areas of Ukraine held by Russian-backed separatists to fund the rebels, which he also denied.

When he escaped house arrest in late February his lawyer said he had faced threats to his life and had been evacuated to a safe place in Kyiv.

His party, Ukraine's biggest opposition grouping in parliament, was among 11 that were suspended last month on security grounds because of alleged links to Russia. Russia condemned the move as a mistake by President Zelensky that would divide the country.

The US slapped sanctions on Mr Medvedchuk in 2014, accusing him of undermining Ukraine's sovereignty. Then this year, the US accused two members of his Opposition Platform party of helping Russia to undermine Ukraine's leadership.

One of those MPs was Taras Kozak, who owned three TV channels that Ukrainian officials controversially shut down last year amid allegations that they broadcast Russian disinformation. Mr Medvedchuk was also closely linked to the TV channels - Zika, 112 Ukraine and Newsone.

Together, the two men are said to have made millions from buying a 42% stake in a Russian oil refinery in the mining city of Novoshakhtinsk for a knockdown price. Mr Medvedchuk eventually bought a majority stake in the refinery in the name of his wife, TV star Oksana Marchenko, to avoid US sanctions.