Russia and Ukraine exchanged prisoners of war under the age of 25 on Monday in emotional homecoming scenes, the first step in a series of planned prisoner swaps that could become the biggest of the war so far.

The exchange, announced by both sides, was the result of direct talks in Istanbul on June 2 that resulted in an agreement to conduct an exchange of at least 1,200 POWs on each side and to repatriate thousands of bodies of those killed in the war.

The return of POWs and the repatriation of the bodies of the dead is one of the few things the two sides have been able to agree on, even as their broader negotiations have failed to get close to ending Russia's war in Ukraine, now in its fourth year.

Fighting has raged on, with Russia saying on Monday its forces had taken control of more territory in Ukraine's east-central region of Dnipropetrovsk and Kyiv saying Moscow had launched its largest drone attack of the war.

Officials in Kyiv said some of the Ukrainian prisoners who came home on Monday had been in Russian captivity since the beginning of the war.

At a rendezvous point for the returning Ukrainian prisoners, soon after they crossed back into northern Ukraine, an official handed one of the freed men a cellphone so that he could call his mother, a video released by Ukrainian authorities showed.

"Hi mum, I've arrived, I'm home!" the soldier shouted into the receiver, struggling to catch his breath because he was overcome by emotion.

In a second video clip distributed by Ukrainian officials, one returned prisoner could be seen talking on the phone to a relative of another man still in captivity.

For relatives of missing Ukrainians, returning prisoners can be the only source of news about their loved ones.

"Don't worry, everything is okay with him," the soldier told the relative, a woman called Tania whose tearful voice could be heard on the other end of the call.

PSYCHOLOGICAL ASSISTANCE

Neither side said how many prisoners had been swapped on Monday, but the Russian Defence Ministry said in its own statement that the same number of military personnel had been exchanged on each side.

Kremlin aide Vladimir Medinsky said at the weekend that a first list of 640 POWs had been handed to Ukraine.

The Russian military said its returned servicemen were now in Belarus, a close Russian ally, where they were receiving psychological and medical assistance before being transferred to Russia for further care.

Footage broadcast by Russia's RIA state news agency showed a group of freed Russian soldiers on board a coach raising their hands in the air and shouting: "Hurrah we're home."

The same group was shown holding a Russian flag and chanting "Russia! Russia!" before boarding the coach.

"It is very difficult to convey what I'm feeling inside now. But I am very happy, proud and grateful to everyone who took part in this process, in the exchange and bringing us home," said one freed Russian soldier.

Ukrainian President Volodymyr Zelenskiy said his country had received a first group of prisoners from Russia and that it would take several days to complete the swap.

Both sides say the intention for this round of prisoner swaps is also to hand over people who are gravely ill or severely injured. The people seen being handed over so far on Monday appeared to be fit and well.

"Today's exchange has begun. It will be done in several stages in the coming days," Zelenskiy said on the Telegram app.

"The process is quite complex, with many sensitive details, and negotiations continue virtually every day. We count on the full implementation of the humanitarian agreements reached during the meeting in Istanbul. We are doing everything possible to bring back every single person."

The Kremlin had said earlier on Monday that Russia was ready to honour agreements with Ukraine on the POW exchange and on the repatriation of dead soldiers despite what it said was Kyiv's failure to fully honour its side of the bargain. Ukraine had denied allegations of postponing the prisoner swaps.