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Kyiv's human rights commissioner on Thursday accused Russians of kidnapping his country's children and selling them for sex.

"Telegram channels revealed that Russians are kidnapping Ukrainian children and making sexual videos with them," Ukrainian parliament's rights commissioner Dmytro Lubinets said in a post on messaging app Telegram.

"For example, they are offering 250,000 rubles (€3,271) for a boy who should start school soon," he said, describing the boy as a Ukrainian orphan.

Lubinets posted an excerpt from an exchange on WhatsApp between two people allegedly discussing using a young boy for a child pornography shoot.

"He has been brought from Ukraine, from an orphanage, has no relatives," one of the two people says in the exchange.

"They ordered a series of videos with him," he said. The person did not give the child's age but said the boy should start school "soon".

"We are bringing in little ones for this type of work, that's not the first we've brought in," the person added. 

Lubinets did not say how he obtained the copy of the exchange but called on Ukraine's police, cyber police and prosecutors to take "appropriate measures to find and punish the guilty.

"The Russian Federation is insidiously kidnapping, killing, deporting and raping our children," he added. 

"How is this even possible in today's world?!"

Ukraine has accused Russian forces of kidnapping thousands of children - including orphans - and taking them into Russian-controlled territory.

In September, UN investigators accused Russia of committing war crimes on a "massive scale" in Ukraine, including torture and sexual violence.

In November, Ukrainian First Lady Olena Zelenska demanded a "global response" to the use of sexual violence as a weapon of war.

She said Kyiv prosecutors were investigating more than 100 possible crimes by Russian soldiers.