Ukraine's President Volodymyr Zelenskiy vowed on Sunday in an Orthodox Easter message that no "wickedness" will destroy the country and prayed that God returns happiness to children and brings solace to grieving mothers.

Standing inside one of the country's best known landmarks, the 1,000-year-old Saint Sophia Cathedral in Kyiv, Zelenskiy said in a video address that Ukraine will overcome the darkness that the war has brought upon it.

"Today, we still believe in the new victory of Ukraine and we are all convinced that we will not be destroyed by any horde or wickedness," Zelenskiy, wearing his trademark dark khaki outfit, said.

"We are overcoming dark times and on this day I - and most of us - are not in bright clothes, but we are fighting for a luminous idea."

Subdued Easter celebrations took place across the country on Sunday, exactly two months since the beginning of the Russian invasion that Russian President Vladimir Putin has called a "special military operation".

Serhiy Gaidai, governor of the eastern Luhansk region, said on Sunday that seven churches there have been destroyed in the war.

While churches used to be full for overnight and morning Easter services, this year churches have been asked not to gather many people, with concerns they could be targets for missiles.

At least 213 children have been killed in the war so far, including a 3-month-old infant in strikes on Saturday in the southern city of Odesa, local officials said.

Russia has denied targetting civilians.

"Give every boy and every girl a happy childhood, youth, and old age, which will allow at least a bit to shed the memories of their terrible childhood during the war," Zelenskiy said.

Zelenskiy said on Thursday Russia had rejected a proposal for a truce over the Easter period.

"Take care of our mothers, give endurance to those who are waiting for a son or daughter to return from the war," he said on Sunday. "Give endurance to those who, unfortunately, would not see the return of their child from the front."

On Saturday, Ukraine said Russia had resumed its assault on the last Ukrainian defenders holed up in a giant steel works in Mariupol.

"Do not forget Mariupol and its heroic defenders," Zelenskiy prayed. "It is possible to destroy the walls, but it is not possible to destroy the foundation on which the spirit of our warriors, the spirit of the whole country, rests."

(Reporting by Natalia Zinets Writing by Lidia Kelly in Melbourne Editing by Frances Kerry)