UNITED NATIONS, United States - The United Nations said Wednesday there are credible accusations that Moscow's forces have removed children from Ukraine to Russia for adoption as part of larger-scale forced relocations and deportations.

"There have been credible allegations of forced transfers of unaccompanied children to Russian occupied territory, or to the Russian Federation itself," Ilze Brands Kehris, the assistant UN secretary-general for human rights, told the Security Council.

"We are concerned that the Russian authorities have adopted a simplified procedure to grant Russian citizenship to children without parental care, and that these children would be eligible for adoption by Russian families," she said.

Brands Kehris told a Security Council meeting on Ukraine that Russian forces are also running a "filtration" operation in which Ukrainians in occupied territories are put through systematic security checks that have involved "numerous" human rights violations.

"In cases that our office has documented, during 'filtration,' Russian armed forces and affiliated armed groups have subjected persons to body searches, sometimes involving forced nudity, and detailed interrogations about the personal background, family ties, political views and allegiances of the individual concerned," she said.

The filtration procedures involved probing people's mobile devices, obtaining personal identity data, and taking pictures and fingerprints, she said.

Some Ukrainians judged as close to the Ukraine government or military have been tortured and forcibly removed and sent to Russian penal colonies and other detention centers, she said.

"We are particularly concerned that women and girls are at risk of sexual abuse during 'filtration' procedures," she said.

Russian UN Ambassador Vasily Nebenzya rebuffed the allegations, calling them "unfounded" and saying the Ukrainians were fleeing their country "to save themselves from the criminal regime," Moscow's reference to the Kyiv government.

He said what was labelled "filtration" was simply registering people coming to Russia.

"As far as we can judge similar procedures are applied in Poland and other countries of the European Union against Ukrainian refugees," he told the Security Council.

Earlier the US State Department said Russian President Vladimir Putin's office is directly managing the filtration program and the forced relocation of thousands of Ukrainians into Russia.

"Russia has systematically used the practice of forced deportations previously, and the fear and misery it evokes for people forced to live under the Kremlin's control are hard to overstate," said State Department deputy spokesman Vedant Patel.

"We assess that the Kremlin use filtration operations as crucial to their efforts annex areas of Ukraine under their control."