The US State Department says it is deeply concerned about the arrest of a Hong Kong online radio host under the national security law last weekend, describing the move as an "assault on human rights and press freedom".

Wan Yiu-sing, who is also known Giggs, was arrested along with his wife and an assistant in North Point on Saturday.

Police said the three were arrested on suspicion of assisting people to commit secession as well as money laundering, adding that the alleged case involves crowd-funding to support people who have left Hong Kong.

A post on Wan's Facebook page said the accusations are linked to a fundraising campaign to support young Hong Kong protesters living and studying in Taiwan.

"The US is deeply concerned about the arrests," said Cale Brown, deputy spokesman of the US State Department.

"Another example of the Chinese Communist Party and their Hong Kong proxies' assault on human rights and press freedom," he wrote on Twitter.

Police released the trio on bail without charging them. Wan, from the internet radio channel D100, has since said he will not be broadcasting for a while.