The US Federal Trade Commission (FTC) has referred a complaint against TikTok and its Chinese parent company ByteDance over potential violations of children's privacy to the Department of Justice (DOJ).
The FTC says its own investigation "uncovered reason to believe" that the firms "are violating or are about to violate the law".
In a statement to BBC News, a TikTok spokesperson said they were disappointed by the decision.
The case is separate from legislation passed earlier this year to ban TikTok in the US if ByteDance does not sell the business.
The regulator said its investigation focused on potential violations of the FTC Act and the Children's Online Privacy Protection Act (COPPA).
The FTC also said it does not usually announce that it has referred a complaint to the DOJ but in this instance felt doing so was in the public interest.
COPPA governs the collection, use and disclosure of personal information by online services about children under 13-years-old.
The FTC Act targets “unfair or deceptive acts or practices” by companies.
In response, a TikTok spokesperson said the company disagreed with the allegations and that it had "been working with the FTC for more than a year to address its concerns."
"We're disappointed the agency is pursuing litigation instead of continuing to work with us on a reasonable solution," they added.
A DOJ spokesperson told BBC News they "cannot comment on the substance of the referral from the FTC against TikTok."
"Consistent with our normal approach, the Justice Department consulted with FTC in advance of this referral and will continue to do so as we consider the claims," they added.
The FTC's announcement adds to the growing pressure faced by TikTok in the US.
In April, President Joe Biden signed into law a bill that gave ByteDance a maximum of a year to sell the app or face a ban in the country.
That means the deadline is likely to come some time in 2025, after the winner of the 2024 presidential election takes office.
The law was introduced to address concerns that TikTok might share user data with Chinese authorities - claims the company has denied.
In May, TikTok filed a lawsuit aiming to block that legislation, arguing it is an "extraordinary intrusion on free speech rights" of the company and its 170 million American users.
Source: BBC