The US government has filed a new lawsuit against TikTok, accusing the social media company of unlawfully collecting children's data and failing to respond when parents tried to delete their children's accounts.

The Department of Justice said the firm's practices amounted to a "massive-scale" invasion of child privacy, in defiance of laws that require online companies to obtain parental consent to gather information for those under the age of 13.

Officials said the firm, and parent company ByteDance, were "repeat offenders", noting a similar case brought in 2019.

TikTok disputed the claims and said it was "proud" of its efforts to protect children.

It said it proactively removed underage users and had voluntarily put in place protections, such as default screen-time limits and additional privacy guards for minors.

"We disagree with these allegations, many of which relate to past events and practices that are factually inaccurate or have been addressed," a spokesperson said.

"We are proud of our efforts to protect children, and we will continue to update and improve the platform."

The complaint ramps up the pressure on TikTok and Chinese parent company ByteDance, which were already battling the US over a law that threatens to ban the company from the country.

In the filing, the US accused TikTok of implementing "deficient" ways of screening for accounts created by children and obstructing parents when they tried to delete them.

A review of 1,700 requests from parents to cancel accounts that were made between 2019 and 2020 found that 30% were still active in November 2021, according to the filing.

It said moderators charged with reviewing accounts flagged as underage were only authorised to act if they saw explicit admission of a child's age, while being given limited access to user videos.

It said they typically had only a few seconds to make their decisions.

Officials asked the court to order a stop to TikTok's actions and determine penalties for each violation.

"This action is necessary to prevent the defendants, who are repeat offenders and operate on a massive scale, from collecting and using young children’s private information without any parental consent or control,” said principal deputy assistant attorney general Brian Boynton, head of the Justice Department’s Civil Division.

TikTok is among the most popular social media platforms in the world, claiming more than 1 billion users globally and more than 170 million in the US.

Surveys by Pew Research have found that more than 60% of US teens aged 13-17 use the platform, more than half of them on a daily basis.

The lawsuit states the US government seeks "to put an end" to TikTok’s alleged "unlawful massive-scale invasions of children’s privacy".

The company has faced fines in the UK and Europe over similar issues.