Testimonies of Russian soldiers taken prisoner and documents obtained indicate that Russia is preparing a brutal and cynical invasion of Ukraine in violation of the rules of warfare, Ukrainian President Volodymyr Zelensky has said.

"The Russian military, taken prisoner by our defenders, spoke... We heard their testimony. We saw documents, maps, plans. Not yesterday. This is not improvisation. They prepared this invasion just like that – cruelly, cynically. Knowingly in violation of the rules of warfare," the head of the Ukrainian state said in a televised address on Sunday.

Zelensky said that this is about hundreds, hundreds of prisoners, including pilots of planes that bombed Ukrainian cities and civilians.

"That's why Kharkiv, Chernihiv, Sumy, Mariupol and many, many other Ukrainian hero cities saw pure evil, atrocity. It was planned that way," the President of Ukraine said.

He said that this would not kill the humanity of the Ukrainians.

"Despite everything, we treat prisoners within the framework of the Geneva Convention. Despite everything, our missiles do not fall on civilian targets in Russia or Belarus in response, from where the rockets are coming into our territory every day, from where columns of equipment are leaving, aviation is flying against peaceful people, against peaceful cities," Zelensky said.