A report by the Organization for Security and Cooperation in Europe (OSCE) expresses "grave concerns" of alleged mistreatment by the Russian authorities of tens of thousands of Ukrainians who had been forcibly deported from their country and sent to so-called "filtration centers."
"There are reports indicating that people are subject to harsh interrogations and humiliating body searches in such centers," says a 115-page report seen by AFP.
The report calls the establishment of such centers an "alarming" development.
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It adds that those found to have collaborated with Ukrainian authorities "often simply disappear," with some allegedly being transferred to Russian-controlled territories, where they are detained or even killed.
"Signs of torture and ill-treatment on the corpses of killed civilians also show disregard of the principle of humanity," it says.
The report was based on information obtained by three experts named by the OSCE covering the Ukraine war from April 1 to June 25.
The Russian Embassy in Washington called the comments an attempt to stoke "Russophobia" and "poor-quality Western disinformation."
"Washington's attempt to vilify the armed forces of the Russian Federation is apparently connected with dissatisfaction with the success of a special military operation," the embassy said in an online post. Moscow refers to its invasion of Ukraine as a "special military operation."
A previous OSCE mission, covering the start of Russia's invasion on February 24 through April 1, found "clear patterns of international humanitarian law violations."
Human Rights Watch (HRW) also expressed concerns about the reports, saying Russia must immediately disclose the whereabouts of and release all illegally detained or deported Ukrainians.
"Ukrainian civilians detained by Russian troops not only lose their freedom, but also face threats to their health and lives because they are being held without legal or public control," said Tatyana Lokshina, deputy Europe and Central Asia director at HRW.
The comments come after U.S. Secretary of State Antony Blinken on July 13 accused Russia of forcibly deporting hundreds of thousands of Ukrainians from areas it controls in the east and south of the country to Russia.
Blinken said an estimated 900,000 to 1.6 million Ukrainian citizens, including 260,000 children, have been interrogated, detained, and deported from their homes into Russia, including to isolated areas in the Far East, through "filtration" operations.
He called on Russia to stop these operations, which he said violate the Geneva Conventions.