Ukrainian President Volodymyr Zelenskiy's chief of staff thanked a Vatican envoy on Tuesday for his efforts in trying to bring home thousands of children forcibly taken to Russia and asked him for help in returning prisoners of war.

Cardinal Matteo Zuppi has visited both Russia and Ukraine on a mission mandated by Pope Francis to try to help end the war in Ukraine, now more than two years old.

He said after the visits that his talks focused mainly on humanitarian issues rather than a specific peace plan. He also visited China to facilitate prospects for peace.

Presidential chief of staff Andriy Yermak, quoted on Zelenskiy's website, told Zuppi by telephone that Ukraine had devoted considerable diplomatic efforts to securing the return of children taken to Russia.

"You are continuing to work on this matter. Thank you," Yermak was quoted as saying. "This process is proceeding slowly, but it is still moving ahead."

Ukrainian Ombudsman Dmytro Lubinets last week handed to Qatar, which has acted as an intermediary, a list of 561 Ukrainian children whose return Kyiv was seeking from Russia.