A long-awaited exchange of prisoners between Moscow and Kiev has begun, Russian state television reported on Saturday, broadcasting footage of buses leaving a jail in the capital."Buses have left the Lefortovo jail within the framework of preparations for a prisoner exchange," the Rossiya 24 rolling news channel said. AFP correspondents at the scene saw two buses with tinted windows leaving the high-security prison in Moscow under a police convoy.On Thursday, Russian President Vladimir Putin said for the first time the "large-scale" prisoner exchange with Ukraine was being finalised.The Russian leader said the swap would be "a huge step towards normalising relations" following comedian Volodymyr Zelensky's rise to power in Kiev in May.The Russian side found it difficult to agree to the names Ukraine had put forward for the swap, Putin added.Last week media reports said a prisoner exchange between the two countries was imminent and some Ukrainian prisoners had been moved to Moscow from their jails. The apparent preparations then stalled.It is unclear who will be part of the swap and Moscow has been tight-lipped.Film director and activist Oleg Sentsov, 43, has become Ukraine's most famous political prisoner. He was arrested in 2014 and has been serving a 20-year sentence in a Russian Arctic penal colony for planning "terrorist attacks" in Moscow-annexed Crimea.Among other prisoners who could be eligible for a swap are 24 Ukrainian sailors captured last year.Russia has been holding the sailors since seizing three vessels off Crimea last November in the most dangerous direct clash between Russia and Ukraine in years.Some 13 000 people have been killed in Ukraine's conflict with Russian-backed separatists in eastern Ukraine, which broke out shortly after Moscow annexed Crimea in 2014.