The stinging legal defeat for TikTok sent officials scrambling for a solution with the Biden White House saying it will not enforce the ban, which comes into force one day before Donald Trump takes office.

The court unanimously ruled that the law does not violate free speech rights and that the US government had demonstrated legitimate national security concerns about a Chinese company owning the app.

The justices were considering a law Congress overwhelmingly passed a law last year forcing ByteDance to either sell the platform or close it in the United States by January 19.

ByteDance has categorically refused to consider a sale of TikTok, a position supported by the government in Beijing that has called the law a robbery.

"There is no doubt that, for more than 170 million Americans, TikTok offers a distinctive and expansive outlet for expression, means of engagement, and source of community," the justices said.

"But Congress has determined that divestiture is necessary to address its well-supported national security concerns regarding TikTok's data collection practices and relationship with a foreign adversary," they concluded.

With the court's decision, Sunday's ban effectively stands even if lawmakers and officials across the political spectrum were calling for some sort of delay.

The White House said it will not enforce the ban and will leave the matter up to President-elect Donald Trump, who takes office a day later.

Trump also opposes the ban and said that he discussed TikTok in a phone conversation with Chinese President Xi Jinping on Friday.

"The Supreme Court decision was expected, and everyone must respect it," Trump said on Truth Social, asking for time to enforce the ruling.

In a potential signal of support for a delay, the Department of Justice, which will be charged with enforcement, in a statement said enacting the law "will be a process that plays out over time."

- 'Viable deal' -

Speaking to Fox News on Thursday, Trump's incoming national security advisor Mike Waltz said the administration would put in place measures "to keep TikTok from going dark."

He noted that the law allowed for a 90 day delay if the White House could demonstrate that "a viable deal" was in the works.

Frank McCourt, the former owner of the Los Angeles Dodgers who has expressed interest in buying TikTok, said he was "ready to work with the company and President Trump to complete a deal."

As written, the law directs Apple and Google to remove TikTok from their app stores, preventing new downloads.

If not compliant, those companies and internet service providers could pay penalties as high as $5,000 per user able to access the app.

In his arguments to the Supreme Court, TikTok lawyer Noel Francisco stated the site would go dark on Sunday if the justices failed to block the ban, and a media report said the company was planning a full suspension of service in the United States.

TikTok has been lobbying furiously to thwart the law's implementation with CEO Shou Chew set to attend Trump's inauguration on Monday as he fights to save the business.

"Content creators on TikTok are preparing for a possible ban in a context of great uncertainty," Courtney Spritzer, head of digital marketing agency Socialfly, which specializes in social networks, told AFP.

As for advertisers, "some are betting there will be a shutdown while others are more optimistic that it will continue to exist after Sunday," she added.