China yesterday accused the United States of "bullying" over popular video app TikTok, after President Donald Trump ramped up pressure for its US operations to be sold to an American company.

In the latest diplomatic spat between the world's two biggest economies, Beijing hit back after Trump gave TikTok six weeks to arrange a sale of its US operations -- and said that his government wanted a financial benefit from the deal.

"This goes against the principles of the market economy and the (World Trade Organization's) principles of openness, transparency and non-discrimination," said foreign ministry spokesman Wang Wenbin.

"It's outright bullying."

The app has been under formal investigation on US national security grounds, and Trump said that Microsoft was in talks to buy TikTok.

He has given ByteDance until mid-September to strike a deal, a tactic that is almost unheard of.

"It's got to be an American company... it's got to be owned here," Trump said on Monday. "We don't want to have any problem with security."

The US and China have clashed over various fronts recently, essentially barring Chinese telecoms company Huawei from the US market and waging a global campaign to isolate the company over national security concerns.

TikTok has as many as one billion worldwide users, who make quirky 60-second videos with its smartphone app.

But the pressure for a sale of its US and international business, based in Los Angeles, has left the company and its Chinese parent ByteDance facing tough decisions.

In an earlier statement on Monday, ByteDance said it has always been committed to becoming a global company and was considering "re-establishing TikTok headquarters in major markets outside the United States".

UK media reported that it was considering a relocation to London.