Artists have painted brilliant sunflowers over heaps of burnt-out cars destroyed in Russia's invasion of Ukraine - upsetting some locals who wonder if it is too soon to beautify the wreckage of war.

The group of painters from Ukraine and the United States say they plan to sell digital images of the work as non-fungible tokens (NFTs) and raise money for Ukrainian artists, rebuilding projects and other causes.

The cars were largely recovered in the city of Irpin, on the outskirts of the capital, from a bridge destroyed by Ukrainian forces to halt the advance of Russian tanks, Trek Kelly, a Los Angeles-based muralist who helped initiate the project, said.

City authorities had approved the work and assured the artists no one had died in the vehicles, he added.

One couple who owned one of the vehicles had thanked them "for repurposing these cars into something more beautiful," Kelly told Reuters.

Others were less sure as they walked around the work this week on the main road leading into Irpin, where authorities say 200-300 civilians were killed by Russian attacks before the city was taken back by Ukrainian forces in late March.

Russia has denied targeting civilians in what it calls its "special military operation" in Ukraine.

"I understand the idea of the flowers showing hope for the future, and that Ukraine cannot be destroyed despite what the Russians tried to do here, but maybe it's too soon," said Casimir Kiendl, who originally hails from Wales but was living in Ukraine when the war started.

"The memories are still super fresh," said Kyiv resident Yuliya Zaliubovska, who fled to France during the war and stopped for a look during a visit back in Ukraine on Wednesday.

Kelly and Olena Yanko - a Ukrainian artist involved in the project - said they respected the concerns, but hoped the site would become a place for reflection.

"Yes, there are people who didn't understand us. They think that we are dancing on the graves of those who died," Yanko said.

"But we want to show that ... life will go on, we will win (the war) and we can beat the enemy, whether it's with a paintbrush or with weapons."

U.S. charity beautifyearth.org is accepting tax-deductible donations for the artists, and details will be posted soon on where to buy NFTs, Kelly said.

Other cities had already offered them sites for more murals with sunflowers, the Ukrainian national flower, he added.

"They want to beautify these distressed areas until they can be rebuilt so ... there's some brightness and colour and nature springing up out of the ruins in a type of rebirth."