Rapes and sexual assaults attributed to Moscow's forces in Ukraine are part of a Russian "military strategy" and a "deliberate tactic to dehumanise the victims", UN envoy Pramila Patten told AFP in an interview.

"All the indications are there," the UN special representative on sexual violence told AFP when asked if rape was being used as a weapon of war in Ukraine.

"When women are held for days and raped, when you start to rape little boys and men, when you see a series of genital mutilations when you hear women testify about Russian soldiers equipped with Viagra, it's clearly a military strategy," she said.

"And when the victims report what was said during the rapes, it is clearly a deliberate tactic to dehumanise the victims."

The United Nations has verified "more than a hundred cases" of rape or sexual assaults in Ukraine since Russia invaded in February, Patten said, referring to a UN report released in late September.

The report "confirmed crimes against humanity committed by the Russian forces, and according to gathered testimonies, the age of the victims of sexual violence ranges from four to 82 years old," she said.

The victims are mostly women and girls, but also men and boys, she added.

But "reported cases are only the tip of the iceberg," she added.

"It's very difficult to have reliable statistics during an active conflict, and the numbers will never reflect reality, because sexual violence is a silent crime" that is largely underreported, she said.