Washington DC - President-elect Donald Trump on Sunday called for the US to take part-ownership of TikTok and vowed to issue an executive order to delay a ban on the wildly popular app, buying time to "make a deal."

Trump's announcement came hours after TikTok went dark in the US under a law banning it in the name of "national security," as a deadline for its Chinese owners ByteDance to sell its US subsidiary to non-Chinese buyers lapsed.

The law however includes a clause allowing for a 90-day delay if the White House can show progress toward a viable deal, but so far ByteDance has flatly refused any sale, despite last-minute offers.

Outgoing President Joe Biden's administration said it would leave the matter to Trump – who takes office on Monday – and that it would not enforce any ban.

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I'm asking companies not to let TikTok stay dark!" Trump said on his Truth Social platform. "I will issue an executive order on Monday to extend the period of time before the law's prohibitions take effect, so that we can make a deal to protect our national security."

He said he "would like the United States to have a 50% ownership position in a joint venture," arguing that the app's value could surge to "hundreds of billions of dollars – maybe trillions."

"By doing this, we save TikTok, keep it in good hands," Trump wrote.

In an earlier message to dismayed users trying to access the app on Sunday, TikTok said that "we are fortunate that President Trump has indicated that he will work with us on a solution to reinstate TikTok once he takes office. Please stay tuned!"

Trump, who had previously backed a TikTok ban and during his first term in office made moves towards one, did not offer further details on how such a deal would work.

Sunday's blackout came after the Supreme Court on Friday upheld the legislation banning it pending any sale.

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But the app now has a fan in Trump, who pulled a U-turn after initially signing an executive order stepping up pressure on ByteDance to sell in 2020.

It is unclear what the incoming president can do to lift the ban unless ByteDance sells, however.

"Congress wrote this law to be virtually president-proof," warned Adam Kovacevich, chief executive of industry trade group Chamber of Progress.