"My Administration has been working very hard on a deal to save TikTok, and we have made tremendous progress," Trump said on Truth Social, just hours before the deadline was to expire.

"A transaction requires more work to ensure all necessary approvals are signed, which is why I am signing an Executive Order to keep TikTok up and running for an additional 75 days."

The hugely popular video-sharing app, which has more than 170 million American users, is under threat from a US law passed last year that orders TikTok to split from its Chinese owner ByteDance or get shut down in the United States.

Trump has insisted his administration is near a deal to find a buyer for TikTok and keep it from shutting down that would involve multiple investors, but has given few details.

ByteDance, while confirming that it was in talks with the US government towards finding a solution, warned that there remained "key matters" to solve.

"An agreement has not been executed" and whatever was decided would be "subject to approval under Chinese law," the company added.

Motivated by national security fears and belief in Washington that TikTok is controlled by the Chinese government, the ban took effect on January 19, one day before Trump's inauguration, with ByteDance having made no attempt to find a suitor.

TikTok temporarily shut down in the United States and disappeared from app stores, to the dismay of millions of users.

But the Republican president quickly announced an initial 75-day delay and TikTok was restored to users, returning to the Apple and Google app stores in February.

The new 75-day delay pushes the deadline to June 19.

Trump has repeatedly downplayed risks that TikTok is in danger, saying he remains confident of finding a buyer for the app's US business.

The president added on Friday that he would "continue working in good faith with China," whose government will need to sign off on the transaction.

The president suggested TikTok could even be part of a broader deal with China to ease the stinging tariffs he imposed on Beijing as part of a worldwide blitz of levies.

"We do not want TikTok to 'go dark.' We look forward to working with TikTok and China to close the deal," he added.

According to reports, the solution in the works would see existing US investors in ByteDance roll over their stakes into a new independent global TikTok company.

Additional US investors, including Oracle and Blackstone, the private equity firm, would be brought on to reduce ByteDance's share in the new TikTok.

Much of TikTok's US activity is already housed on Oracle servers, and the company's chairman, Larry Ellison, is a longtime Trump ally.

ABC News reported on Friday that Walmart was also in the mix, spurred on by a late expression of interest by retail archrival Amazon to buy the app.

Walmart and Oracle were previously rumored to be buying TikTok in the US when Trump tried to wrest the company from its Chinese owners during his first administration.

Trump long supported a ban or divestment, but has lately defended TikTok, seeing it as a reason more young voters supported him in November's election.

- What about the algorithm? -

Uncertainty remains, particularly over what would happen to TikTok's valuable algorithm.

"TikTok without its algorithm is like Harry Potter without his wand -- it's simply not as powerful," said Forrester Principal Analyst Kelsey Chickering.

Various media reports suggest the new company could license the algorithm from ByteDance, which would remain invested in TikTok.

But such an arrangement would go against the spirit of the law, which is in part based on the premise that TikTok's algorithm can be weaponized by the Chinese against US interests.