Ukraine’s security service said on Tuesday that agents detained Viktor Medvedchuk, a politician and oligarch who has been seen as the Kremlin’s main influential agent in Ukraine in recent years.
Mr Medvedchuk disappeared shortly after the start of the war and the Ukrainian authorities were looking for him.
Ivan Bakanov, the head of Ukraine’s security service, said on Facebook that agents carried out a “lightning fast and dangerous multi-level special operation” to detain Mr Medvedchuk.
Kremlin spokesman Dmitri S. Peskov said he could not confirm that Mr Medvedchuk had been arrested, according to Russian state media. Ukraine’s President Volodymyr Zelensky posted a photo on Telegram in which Mr Medvedchuk appeared to be in handcuffs.
The arrest of Mr Medvedchuk would be a serious blow to President Vladimir V. Putin of Russia, who is said to be the godfather of Mr Medvedchuk’s daughter and considers him one of his closest allies in Ukraine.
He was for a long time an influential player in the politics and business between Ukraine and Russia. He was an adviser in President Leonid Kuchma’s office after Ukraine’s independence and participated in negotiations with separatists in eastern Ukraine after Russia’s annexation of Crimea.
According to local media, Mr Medvedchuk was under house arrest before the war on charges of treason, attempted looting of national resources and aiding and abetting a terrorist organization.
A Ukrainian official, Mykhailo Podolyak, a member of Ukraine’s delegation for peace talks in Istanbul, said on Twitter that Mr Medvedchuk had “regularly lied about the situation” in Ukraine, and “stole money and eventually became one of the initiators of the war”.
Oleksandr Chubko contributed from Kiev, Ukraine.