Russian forces have been forcibly transferring Ukrainian civilians, including those fleeing hostilities, to areas under their control, Human Rights Watch said in a report.
Forced transfers "are a serious violation of the laws of war amounting to a war crimes and a potential crime against humanity," the non-governmental organisation said.
HRW interviewed 54 people who went to Russia or knew people who did. Some of them were also helping Ukrainians trying to leave Russia after Moscow's forces invaded Ukraine at the end of February.
Many of the people forcibly transferred were fleeing the city of Mariupol, a port that suffered a devastating siege and heavy shelling before being seized by Russian troops.
Others were from the Kharkiv region in eastern Ukraine.
"Of course, we would have used the opportunity to go to Ukraine if we could have," one woman transferred from Mariupol told HRW.
"But we had no choice, no possibility to go" to Ukrainian-held territories, she said.
"Ukrainian civilians should not be left with no choice but to go to Russia," Belkis Wille, senior crisis and conflict researcher at Human Rights Watch and co-author of the report, said.
On the way, many were subjected to a form of compulsory security screening called "filtration" that included the collection of biometric data and fingerprints, body searches, and searches for personal belongings, according to the report.
"No-one should be forced to undergo an abusive screening process to reach safety," Ms Wille said.
A man from Mariupol told HRW that, after being detained by Russian troops, he and dozens of Mariupol residents were held in a village schoolhouse for two weeks under filthy conditions, before undergoing filtration.
"We felt like hostages," he told HRW.
The filtration process - in its scope and the systemic manner targeting Ukrainian citizen - is "punitive and abusive," according to Human Rights Watch.
It has "no legal underpinning" and is a "clear violation of the right to privacy," according to the NGO.
HRW sent its findings and a summary of questions to the Russian authorities on 5 July but received no response.
"Herding people further into Russian-occupied areas and onward to Russia without consent should immediately stop," Ms Wille said.
Separately, the Red Cross said its officials have failed to secure access to Ukrainian prisoners of war held in the Russian-controlled town of Olenivka where dozens were killed in an attack in July.
Ukraine and Russia have traded accusations over the missile strike or explosion in the front-line town of Olenivka in eastern Donetsk that killed prisoners held by Moscow-backed separatists.
International Committee of the Red Cross Director-General Robert Mardini told reporters in Kyiv that the group was engaged in intense negotiations with Russian authorities, but had not been granted access and also lacked security guarantees to carry out such a visit.
The Red Cross registered 1,800 people taken from the besieged Azovstal steel works in the Ukrainian port of Mariupol, with the understanding that it would be allowed to visit them, but that has not been possible, he told a news conference.
Olenivka is about 90km north of Mariupol.
Mr Mardini said the Red Cross had visited hundreds of POWs on both sides of the war, now in its seventh month, but there were thousands more it still had not been able to see.
"We are negotiating every day to have full access to all prisoners of war," Mr Mardini said. "It is clearly an absolute obligation (of) the parties to give the ICRC access to all prisoners of war."
He declined to provide any details about conditions in the prisons that had been visited.
Mr Mardini said the group was discussing the lack of access to prisoners at the Olenivka site with officials at "every level" of the Russian government and was engaged in a "very constructive" dialogue with the Ukrainian government about what he called an "unsatisfactory" situation.
He said the Red Cross had facilitated the transport of over 1,000 letters from Ukrainian troops being held in Russia to Ukraine, where they will be delivered to their families.
He said the aid group had provided more than 3,000 families with news of their loved ones since the start of the war, which Russia calls a "special military operation".
