Donald Trump says the government should get a cut from the sale of TikTok's US unit if an American firm buys it.

The US president said he made a demand for a "substantial portion" of the purchase price in a phone call at the weekend with Microsoft's boss.

He also warned he will ban the app, which is owned by China's ByteDance, on 15 September if there is no deal.

ByteDance is under pressure to sell its US business after Mr Trump threatened a crackdown on Chinese tech companies.

The Trump administration has accused TikTok and others of providing data to the Chinese government, which Beijing and TikTok deny.

"The United States should get a very large percentage of that price, because we're making it possible," Mr Trump said.

"It would come from the sale, which nobody else would be thinking about but me, but that's the way I think, and I think it's very fair," he added.

The request for payment to the US Treasury further complicated negotiations as legal experts highlighted that such a demand to secure regulatory approval for a takeover deal would be highly unorthodox.

Nicholas Klein, a lawyer at DLA Piper, said generally "the government doesn't have the authority to take a cut of a private deal through" the Committee on Foreign Investment in the United States, which is the inter-agency committee that reviews some foreign investments in the US.