A Russian missile strike on Ukraine's northern city of Chernihiv killed seven people and wounded 144 on Saturday, the country's president Volodymyr Zelensky said, in what the UN denounced as a "heinous" attack.

The strike came during the Orthodox holiday of the Transfiguration of the Lord, as some attended morning church services in the city.

"It is heinous to attack the main square of a large city, in the morning, while people are out walking, some going to church to celebrate a religious day for many Ukrainians," said Denise Brown, the UN's humanitarian coordinator for Ukraine.

"I condemn this repeated pattern of Russian strikes on populated areas of Ukraine... Attacks directed against civilians or civilian objects are strictly prohibited under international humanitarian law," she added.

Audrey Azoulay, director general of the UN's cultural organisation Unesco, said she was "appalled" by the attack, in a post on social media.

Ukraine's culture ministry said the centre of Chernihiv, a city with a thousand-year history, is a candidate for nomination to the Unesco World Heritage List.

The city, 150 kilometres north of Kyiv towards Belarus, had until now been largely spared from major attacks since the first months of Russia's military offensive as fierce fighting rages in the east and south.

The Russian army marched through the city when it assaulted Ukraine through Belarus in February 2022, before being repelled by Kyiv's forces.