Donald Trump said he will move as soon as Saturday to ban TikTok from the U.S., several weeks after the administration first said it was considering taking such an action against the popular Chinese-owned social video app over concerns that it’s a threat to national security.

“As far as TikTok is concerned, we’re banning them from the United States,” Trump said Friday, speaking to reporters on Air Force One. Microsoft was said to be in talks to acquire TikTok from ByteDance, the Beijing-based internet company, but Trump said he opposed such a deal, per NBC News.

Regarding banning TikTok, “I have that authority,” Trump claimed, saying he could use emergency economic powers or an executive order to enforce the decision. TikTok has maintained that it has never provided user data to Chinese authorities and it wouldn’t if the communist regime requested it.

But it’s not clear how a Trump order to “ban” TikTok would work — or if it would stand up in court. The Trump administration might try to threaten to punish Apple and Google if they carry TikTok in their U.S. app stores, by adding TikTok to the Commerce Department’s list of foreign entities that “present a greater risk of diversion to weapons of mass destruction (WMD) programs, terrorism, or other activities contrary to U.S. national security and/or foreign policy interests.”

The U.S. government last year added Chinese telecom equipment manufacturer Huawei to the “entity list,” prompting Google and others to cut off their business with Huawei. But putting TikTok on that list would be unusual and legally dubious, James Lewis, director of technology policy at the Center for Strategic and International Studies (CSIS), recently told The Verge. There’s no evidence TikTok has engaged in criminal activity threatening U.S. national security, although TikTok was fined for alleged violations of the U.S.’s child data-privacy law (which the FTC is reinvestigating).

The Trump administration also could try to invoke the International Emergency Economic Powers Act to impose economic sanctions that would block Apple and Google from publishing the TikTok app. This, however, also has a high legal threshold: The law requires that the president first declare a national emergency (which can be nullified by Congress) in the event of an “unusual and extraordinary threat, which has its source in whole or substantial part outside the United States, to the national security, foreign policy, or economy of the United States.” For Trump to enact a ban via the IEEPA, he would need to explain to Congress why TikTok represents an “unusual and extraordinary” danger, according to the law.

The ACLU slammed Trump’s threat to ban TikTok as harmful to free expression and pointed out that it’s technically “impractical.”

“Banning an app like TikTok, which millions of Americans use to communicate with each other, is a danger to free expression and technologically impractical,” the ACLU tweeted.

With any internet platform, “we should be concerned about the risk that sensitive private data will be funneled to abusive governments, including our own,” Jennifer Granick, the ACLU’s surveillance and cybersecurity counsel, said in a statement Saturday. “But shutting one platform down, even if it were legally possible to do so, harms freedom of speech online and does nothing to resolve the broader problem of unjustified government surveillance.”

The U.S. does not have a nationwide, network-level ability to block internet content or apps, which is how China’s Great Firewall enforces censorship. Like China, India has a national-scale blocking capability, under which it banned TikTok along with dozens of other apps from Chinese companies in late June.

In a statement, TikTok said, “TikTok U.S. user data is stored in the U.S., with strict controls on employee access. TikTok’s biggest investors come from the U.S. We are committed to protecting our users’ privacy and safety as we continue working to bring joy to families and meaningful careers to those who create on our platform.”

Vanessa Pappas, TikTok’s U.S. general manager, shared a video on Saturday morning, saying the app is “not planning on going anywhere” and is “here for the long run.”

“I want to say thank you to the millions of Americans who use TikTok every day, bringing their creativity and joy into our daily lives. We’ve heard your outpouring of support and we want to say thank you. We’re not planning on going anywhere,” she said.

Pappas continued, “TikTok is a home for creators and artists to express themselves, their ideas and connect with people across different backgrounds, and we are so proud of all the various communities that call TikTok home. I’m also proud of our 1,500 U.S. employees who work on this app every day, and the additional 10,000 jobs we’re bringing into this country over the next three years. I’m thrilled about our U.S. Creator Fund, where we just announced our $1 billion fund to support our creators. And when it comes to safety and security, we’re building the safest app because we know it’s the right thing to do. We appreciate the support, we’re here for the long run, continue to share your voice here, and let’s stand for TikTok.”

In 2020 to date, TikTok said it has hired “nearly 1,000 people” in the U.S. The app has surged in popularity, particularly among teens and young adults, for creating and sharing lip-syncing music videos and other short clips. ByteDance bought Musical.ly, the predecessor app to TikTok, in 2017.

Earlier this summer, TikTok was in the headlines after claims by users that they helped depress attendance at Trump’s June 20 rally in Tulsa, Okla., by reserving thousands of tickets. The Trump 2020 re-election campaign claimed that the flood of fake ticket requests had no effect on either crowd forecasts or attendance.

Trump’s claim that he would issue some kind of ban on TikTok — amid an escalating confrontation with China — followed reports Friday that his administration was planning to order ByteDance to divest TikTok, under the Committee on Foreign Investment in the United States (CFIUS).

This week, TikTok CEO Kevin Mayer responded to the U.S. government’s concerns over security and its Chinese ownership, announcing that TikTok will provide greater transparency into its operations by publicly disclosing the app’s algorithms, moderation policies and data flows. He also said the newly established $200 million TikTok Creator Fund to fund American creators on the platform will grow to over $1 billion in the U.S. in the next three years, and more twice that globally.

Some of ByteDance’s American investors, who have been seeking to acquire TikTok, have valued the app provider at $50 billion, according to a Reuters report.