Russia and Ukraine were set to take part in a major prisoner swap on Friday, described as the biggest since the start of the full-scale Russian invasion in 2022.

Officials from both countries agreed to exchange 1,000 prisoners of war each, during otherwise fruitless peace negotiations in Istanbul a week ago.

US President Donald Trump earlier posted his congratulations on his Truth Social platform, claiming that the swap was complete and that “this could lead to something big???”.

However, there was no confirmation from either side, and Ukrainian military sources told news agencies only that the process was under way.

Unconfirmed reports suggested that the exchange could take place over three days across the Ukrainian border with Belarus.

The swap was agreed in Turkey a week ago, when low-level delegations from Ukraine and Russia came face to face for the first time since March 2022, even though the meeting lasted only two hours and failed to make any progress towards a ceasefire.

Russian Foreign Minister Sergei Lavrov said on Friday that there would be a second round of talks, when Moscow would hand a “memorandum” to the Ukrainian side.

Trump said earlier this week that Russia and Ukraine would “immediately” start negotiating towards a ceasefire and an end to the war, after a two-hour phone call with Russian President Vladimir Putin.

Since then, Putin’s Ukrainian counterpart Volodymyr Zelensky has accused him of “trying to buy time” to continue the war.

Italy’s Prime Minister Giorgia Meloni has backed a suggestion from Trump that the Vatican might mediate talks on negotiating a ceasefire, but Lavrov said that was “not a very realistic option”.

The Russian foreign minister repeated an unfounded claim that Zelensky was not a legitimate leader and suggested new elections should be held before a potential future peace agreement is signed.

Asked if Russia was ready to sign a deal, Lavrov said: “First we need to have a deal. And when it’s agreed, then we will decide. But, as President Putin has said many times, President Zelensky does not have legitimacy.”

He said after an agreement was ready, Russia would “see who out of those in power in Ukraine has legitimacy”.

“The key task now is to prepare a peace agreement which will be reliable and provide a long-term, stable and fair peace without creating security threats for anyone. In our case, we’re concerned with Russia.”