US President Donald Trump says he has asked Chinese leader Xi Jinping to "consider" releasing Jimmy Lai, the Hong Kong pro-democracy tycoon who was earlier this week found guilty under the city's controversial national security law.

"I feel so badly," Trump told reporters. "I spoke to President Xi about it and I asked to consider his release."

The UK similarly called for the 78-year-old to be "immediately released", with UK Foreign Secretary Yvette Cooper condemning the ruling, calling it a "politically motivated persecution".

Lai, who is a British citizen, has been in jail since December 2020 and is due to be sentenced early next year. He faces a maximum term of life in prison.

Trump made the brief comment to reporters at the White House on Monday - but did not specify when he had brought Lai's case up to Xi.