Ukraine's President Volodymyr Zelensky on Monday hailed Ukraine's first Oscar, awarded to a documentary about Russia's assault on the city of Mariupol, saying it showed "the truth about Russian terrorism."

The film -- "20 Days in Mariupol" -- won the Best Documentary Oscar at a ceremony in Los Angeles on Sunday night.

Directed by Ukrainian filmmaker Mstyslav Chernov, it depicts intense fighting and near constant Russian aerial bombardment from inside the southern port city of Mariupol in the first days after Russia invaded in 2022.

"Russia brutally attacked Mariupol over two years ago. The film '20 Days in Mariupol' depicts the truth about Russian terrorism," President Volodymyr Zelensky said.

The Kremlin declined to comment earlier on Monday when asked about the film's success.

Zelensky thanked the team behind the production, and said it allowed Kyiv "to speak out loudly about Russia's war against Ukraine."

He also demanded the release of Ukrainian fighters that Russia captured at the Azovstal plant in the city. Dozens of Ukrainian troops had holed up at the vast steelworks in an attempt to stop Russia completing its seizure of Mariupol.

The city, on the Sea of Azov, was almost completely destroyed by Russian attacks that Kyiv says killed tens of thousands of civilians.

The film had already picked up a string of other international accolades, including a Pulitzer Prize and a Bafta.

Success at the world's most prestigious film awards prompted a flood of tributes and emotion across Ukrainian social media.

"A historic and sad event at the same time. Thank you to the makers of this appalling film," pro-Western opposition MP Iryna Gerashchenko said on Facebook.

Accepting the award on Sunday, director Chernov said: "Probably I will be the first director on this stage to say: I wish I'd never made this film."

The award comes with the Ukrainian army struggling on the front lines and vital Western aid wavering.

A $60-billion aid package in the United States, Kyiv's most important military backer, is being held up by political wrangling in Congress.