KYIV, Jan 25 (Reuters) - President Volodymyr Zelenskiy has urged a senior U.N. official to help find a way to resolve what Ukrainian authorities decry as a serious consequence of 11 months of war - the deportation to Russia of thousands of adults and children.

Ukraine has for months denounced reports of mass deportations to Russia, often to remote regions thousands of kilometres from Ukraine. Russia denies any suggestion of mistreatment or criminal intent, describing the mass movements as evacuations.

"The discussion focused above all on our people that the occupiers have deported to Russia," Zelenskiy said in his nightly video address, referring to talks with U.N. High Commissioner for Refugees Filippo Grandi earlier on Wednesday.

"These are adults, these are our children. A mechanism is needed to protect and bring back people and to bring to account all those who are guilty of deportations. I am certain the U.N. institutions can show leadership in resolving this issue."

Zelenskiy described as genocide what he said were Russia's policies of captivity and forcing refugees to take on Russian citizenship. Ukraine's foreign minister has decried the "state-organised kidnapping of children".

Grandi, completing a six-city tour of Ukraine, said he had been particularly moved by the aftermath of a missile strike that killed 46 people last week in the central city of Dnipro.

"I have witnessed war in many places," he said in a video posted on the UNHCR Twitter feed. "But when I see those apartments that have been divided into two pieces ... when I learn that people have been killed in one hit, including six children under the rubble, I am left with little to say."

Zelenskiy has criticised as ineffective the efforts of other international organisations to tackle the issue of the deportations, including the International Committee of the Red Cross and the 57-nation Organisation for Security and Cooperation in Europe.

An official Ukrainian portal called Children of War lists 459 children killed and 916 wounded in the conflict as of Jan. 26. The site says 14,711 children have been deported, and 126 returned.

Ukrainian human rights advocates speak of the possible deportation of hundreds of thousands of citizens.

The U.S. State Department has cited figures of between 900,000 and 1.6 million deported Ukrainians, including 260,000 children.