A major prisoner exchange between Russia, the US and four European countries is under way in Ankara, the Turkish presidency has confirmed.

Three US citizens imprisoned in Russia - Wall Street Journal reporter Evan Gershkovich, US Marine veteran Paul Whelan and Russian-American radio journalist Alsu Kurmasheva - are thought to be among those being released.

In total, the exchange will involve at least 24 prisoners.

Eight Russian nationals are expected to be returned to Russia, including several with suspected ties to Russian intelligence.