Hong Kong pro-democracy newspaper Apple Daily will be forced to shut "in a matter of days" after authorities froze the company's assets under a sweeping national security law, an adviser to jailed tycoon Jimmy Lai told Reuters on Monday.

Mark Simon, speaking by phone from the United States, said Next Digital, publisher of the popular Hong Kong newspaper, will hold a board meeting on Monday to discuss how to move forward.

"We thought we'd be able to make...