Ukraine and Russia began a major prisoner exchange on Friday, which, if completed, would be the biggest swap since the war began more than three years ago.

Both sides received 390 people in this first stage and are expected to exchange 1,000 each in total under an agreement reached at direct talks in Istanbul last week.

Russia has signalled it will send Ukraine its terms for a peace settlement after the swap, which is set to be staggered over three days.

The two enemies have held regular prisoner swaps since Russia launched its 2022 operation – but none have been on this scale.

"The first stage of the '1,000-for-1,000' exchange agreement has been carried out," Ukrainian President Volodymyr Zelensky wrote on X.

"Today – 390 people. On Saturday and Sunday, we expect the exchange to continue."

Russia said it had received 270 Russian troops and 120 civilians, including some from parts of its Kursk region captured and held by Kyiv for months.

The two sides have not yet revealed the identities of those exchanged.

US President Donald Trump earlier congratulated the two countries on the swap.

"This could lead to something big???" he wrote on his Truth Social platform.

Trump's efforts to broker a ceasefire in Europe's biggest conflict since World War II have so far been unsuccessful, despite his pledge to rapidly end the fighting.

After 39 months of fighting, thousands of POWs are held in both countries.

Russia is believed to have the larger share, with the number of Ukrainian captives held by Moscow estimated to be between 8,000 and 10,000.

Kyiv and Moscow have both accused each other of violating the Geneva Convention on the treatment of POWs. The UN said prisoners on both sides had been "subjected to torture and ill-treatment."

With Kyiv not knowing the fate of thousands, each exchange brings surprises, said a senior official.

"Almost every exchange includes people no one had knowledge about," he said.

"Sometimes they return people who were on the lists of missing persons or were considered dead."

Russia said it would send Ukraine its terms for a peace settlement once the exchange was over, without saying what those terms would be.

Diplomatic efforts to end the conflict have stepped up a gear in recent weeks. Russian and Ukrainian officials held their first face-to-face talks in more than three years last week in Istanbul.

But the Kremlin has shown no sign it has walked back its maximalist demands for ending the fighting, which include Ukraine abandoning its Nato ambitions and ceding territory it already controls.

Russia is currently preparing a "draft document outlining the conditions for achieving a sustainable, long-term, comprehensive agreement on the settlement", Foreign Minister Sergei Lavrov said.

"As soon as the exchange of prisoners of war is completed, we will be ready to hand over to the Ukrainian side the draft document that the Russian side is currently finalising," he added.

Trump has been seeking to broker an end to the fighting since taking office in January, but has failed to extract any major concessions from the Kremlin. (AFP)