China accused the United States on Tuesday 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. Beijing slammed the latest move as "political manipulation".

Wang told a regular press briefing Tuesday: "The US, without providing any evidence, has been using an abused concept of national security... unjustifiably suppressing certain non-US companies." He said the national security grounds for the US's clampdown on Chinese firms "does not hold water", adding that the companies conduct their business activities in accordance with international rules and US laws.

"But the US is cracking down on them on trumped-up charges," said Wang, who warned the US not to "open Pandora's box".

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.

'Global company'

In an internal letter to employees on Tuesday, the company's founder Zhang Yiming suggested the US's aim was to ban the app rather than force a sale of its US operations. He told staff to "anticipate more difficulties in the future" and said anti-China sentiment has recently "risen significantly in many countries".

"Regarding public opinion, we must be able to accept misunderstandings for a period of time," he said. "I hope that everyone does not take to heart the short-term reputational damage, and does the right thing with patience."

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. Zhang had earlier said that the company had "always been committed to ensuring user data security, as well as the platform neutrality and transparency".

Meanwhile, Wang urged the US on Tuesday to "refrain from politicising economic issues" and to provide an open, fair and non-discriminatory environment for foreign market players.