The head of the U.N. human rights mission in Ukraine said on Friday that Russia is not allowing access to prisoners of war, adding that the U.N. had evidence that some had been subject to torture and ill-treatment which could amount to war crimes.

Matilda Bogner told a Geneva news briefing that U.N. monitors had unimpeded access to Ukrainian facilities and had documented incidents of torture and ill-treatment of POWs by Ukraine which may also amount to war crimes.

"The Russian Federation has not provided access to prisoners of war held on its territory or in territory under its occupation...," Bogner said.

"This is all the more worrying since we have documented that prisoners of war in the power of the Russian Federation and held by the Russian Federation's armed forces or by affiliated armed groups have suffered torture and ill-treatment."

"In terms of the treatment of prisoners of war, certainly some of the issues could rise to being war crimes - issues of torture and ill treatment of prisoners of war," she said in response to a question about the Russian-held prisoners.

Russia, which invaded Ukraine on Feb. 24, denies torture or other forms of maltreatment of POWs.

It says its forces in Ukraine are engaged in a "special military operation" to disarm the country and remove far-right nationalists it deems a threat to Russia's own security. Ukraine and its Western allies say that is a bogus pretext for an unprovoked war of aggression and that Ukraine poses no threat.

Kremlin spokesman Dmitry Peskov said he could not comment on the U.N. statement because Russian authorities did not have enough information. "We do not know who approached the military and whether they did," he told a news briefing.

Ukraine's General Staff did not immediately respond to a request for comment. Kyiv has previously said it checks all information regarding the treatment of POWs and will investigate any violations and take appropriate legal action.

Ukrainian prisoners are being subject to a "welcoming process" whereby they are forced to walk or run between rows of Russian guards who take turns severely beating them as they enter the facilities, Bogner said. Her team had also received information about Ukrainian prisoners suffering from infectious diseases including hepatitis A and tuberculosis in a penal colony in Olenivka, she said.

At the same briefing, she also urged Russia to release on humanitarian grounds four pregnant prisoners of war being held in Russian-controlled areas of Ukraine.