A tearful Ukrainian President Volodymyr Zelensky attended a memorial service on Saturday to commemorate seven senior officials killed in a helicopter crash, a fresh blow to a nation already grieving its many war dead.

Interior Minister Denys Monastyrskyi, his deputy and five other high-ranking ministry officials were killed on Wednesday when their French-made Super Puma helicopter plummeted amid fog into a nursery near Kyiv.

Another seven people were killed, including one child, in the crash. Officials are still investigating the cause.

"The indescribable sadness is covering the soul," Zelensky wrote in a Telegram post on Saturday.

The helicopter went down just days after at least 45 people were killed in a Russian missile attack that partially levelled a block of flats in the southeastern city of Dnipro.

On the battlefield, Ukrainian forces are fending off an unrelenting Russian onslaught in the east, where Moscow has expended massive resources for incremental advances 11 months into its full-scale invasion.

Zelensky issues a daily video address in which he usually discusses the war, but on Saturday he talked only about the death of Monastyrskyi.

Memorial ceremony for Ukrainian interior minister, his deputy and officials who died in helicopter crash

"My condolences to all relatives and friends. It hurts to think about it, it hurts to talk about it," he said.

"We are losing people every day, whom we will always remember and regret we can't bring back," he added, talking of "how many bright people are taken by wartime".

Zelensky and his wife, Olena Zelenska, paid their respects to the victims' relatives inside the hulking Ukrainian House cultural centre in Kyiv. A crowd of mourners snaked outside toward Independence Square.

Kyrylo Budanov, the military intelligence chief, described Monastyrskyi's death as "a huge loss".

"If not for (Monastyrskyi), everything could have been completely different," he told Ukrainian media, referring to the resistance Ukraine mounted after Moscow's Feb. 24 assault.

"He is a true hero of this country."

Monastyrskyi had been flying to a location near the frontline, a police spokesperson said on Thursday.