Among them is two-year-old Tymofii.

His father Denys Chumak was captured while in the besieged southeastern port city of Mariupol in May 2022, which has witnessed some of the fiercest fighting of the war so far.

Ukrainian security forces, including Denys, struggled for months to repel the invading Russians - before Kyiv ordered them to surrender when further defense became impossible.

The small Chumak family had made the city their home in 2020, and Tymofii was born there.

He was eight months old when he and his mother fled. Denys stayed to fight with the National Guard.

As the months went on Denys ended up defending the Azovstal steel works, just as Russian troops encircled the city.

"Denys and I dreamt that he would come back before Tymofii turned one. But his first birthday went by without Denys, Tymofii made his first step without him, said his first word without him, and, unfortunately, this continues. Tymofii has learned a lot already, a lot of time has passed already, but, unfortunately, we are still waiting."

In the hope of fostering a father-son bond, 25-year-old Alisa tries to show him photos of Denys.

"He recognizes Denys in photographs, so when I ask him where his father is, he simply points at the photograph. The last time Tymofii saw his father he was eight months old, now he is two years and six months old, and his father is just a photograph."

But it's not easy for Alisa either, who recalls the last moments the family were all together.

"I can still see this scene unfolding in front of my eyes, it reminds me of the horrible war movies I used to watch. On that day, he gave me some diapers, some food, storage batteries so that we could light up the basement; he didn't even have time to hug me, he just gave me everything, turned around, said he loved us, and that was it. I saw him getting into the car, and that was it. That was the last time we saw each other."

The next time Alisa saw Denys was in a video from the Olenivka detention facility in Russian-controlled Ukraine in the summer of 2022.

She says he was moved to Russia that autumn.

Alisa again spotted her husband on a Russian Telegram channel in summer 2023.

She says he was recognizable but "looking 10 years older." She has not seen him since.

Ukraine says there are around 8,000 people - civilians as well as military personnel - who remain in Russian captivity as a result of Moscow's full-scale invasion that began two years ago this month.

Ukrainian authorities say about 50 prisoner exchanges involving some 3,000 people, mostly from the armed forces, have taken place since the war began.

President Volodymyr Zelenskiy said the majority of the POWs brought home had taken part in the defense of Mariupol.