US First Lady Melania Trump said Friday she had secured the release of Ukrainian children displaced because of the ongoing war after establishing an extraordinary back channel of communication with President Vladimir Putin.

In a rare public announcement at the White House, she revealed weeks of behind the scenes diplomacy with the Kremlin chief after he held a summit in Alaska with her husband, US President Donald Trump.

Eight children displaced by Moscow's 2022 war on Ukraine had been returned to their homes in the last 24 hours, she said.

The 55-year-old said that Putin had agreed to help after she passed him a letter through Trump at the summit, a meeting which otherwise ended without a breakthrough in resolving the war now in its fourth year.

"Much has unfolded since President Putin received my letter last August. He responded in writing signalling a willingness to engage with me directly and outlining details regarding the Ukrainian children residing in Russia," she told reporters.

"Since then, President Putin and I have had an open channel of communication regarding the welfare of these children."

"My representative has been working directly with President Putin's team to ensure the safe reunification of children with their families between Russia and Ukraine," she said.

"In fact, eight children have been rejoined with their families during the past 24 hours."

Rare glimmer of progress

Seven of them were returned to Ukraine from Russia, she said, while one young girl went back to Russia from Ukraine.

Three were separated from their parents and "displaced to the Russian Federation by frontline fighting," she said.

The others including the girl returned to Russia were "separated from family members across borders" by the conflict.

Russia has said it moved some Ukrainian children from their houses or orphanages for protection due to the threat of hostilities.

Melania Trump said in her announcement that Russia had "demonstrated a willingness" to share details to identify the children, including biographies and photos.

She said she would continue to work for more children to return to their homes.

"This is an important initiative for me. It is built on shared purpose and lasting impact," she added.

The announcement marked a rare glimmer of progress in the Ukraine war, which Trump vowed to solve within 24 hours of taking office but now admits is the most difficult conflict he has tried to solve.