Ukraine and Russia have begun a large-scale prisoner exchange which, if completed, will be the largest since Moscow invaded the country more than three years ago.

In the first stage on May 23, the two sides received 390 people each, and are expected to exchange a total of 1,000 people under an agreement reached in direct talks in Istanbul last week.

The process will continue for three days, Kiev said.

The two countries have been conducting regular exchanges since Russia launched its attack in 2022, but none have been on such a scale.

"The first stage of the '1,000 for 1,000' exchange agreement has been completed. Today, 390 people. On Saturday and Sunday (May 24-25), we expect the exchange to continue," Ukrainian President Volodymyr Zelensky said in X.

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

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

Earlier, US President Donald Trump congratulated both sides on the exchange.

"This could lead to something big???," he wrote in a post on Truth.

Trump's efforts to broker a ceasefire in Europe's deadliest conflict since World War II have so far been fruitless, despite his promise to quickly end the fighting.

After 39 months of fighting, thousands of prisoners of war are being held on both sides.

Russia is believed to hold the larger share, with the number of Ukrainian prisoners held by Moscow estimated at between 8,000 and 10,000.

Kiev and Moscow accuse each other of violating the Geneva Convention on the treatment of prisoners of war, and the UN has said that prisoners on both sides are "subjected to torture and ill-treatment."

Shortly before the exchange, Kyiv issued a statement accusing the Russian army of executing around 270 Ukrainian prisoners of war since the start of the invasion.

Russia regularly violates international norms by prosecuting prisoners of war—there are widespread allegations of torture, and the deaths of several Ukrainian prisoners in custody have been confirmed.

Russian forces are believed to have taken an unknown number of Ukrainian civilians to Russia during the three years of occupation of Ukrainian cities.

There are several well-known cases of Ukrainian civilian prisoners.

This year, Moscow returned the body of journalist Viktoria Roshchina, who died in captivity.

Ukrainians facing trial have told Russian courts that they have experienced and witnessed torture in Russia's notorious prison system. | BGNES