BEIJING, CHINA -- China accused the United States on Tuesday of "outright bullying" over popular video app TikTok, after U.S. President Donald Trump ramped up pressure for its U.S. operations to be sold to an American company.

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

A day earlier, President Trump gave TikTok six weeks to sell its U.S. operations, in the latest escalation to an ongoing political and trade battle between Washington and Beijing.

The app has been under formal investigation on U.S. national security grounds, because it collects large amounts of personal data on users and is legally bound to share it with authorities in Beijing if they demand it.

But Wang told a regular press briefing Tuesday: "The U.S., without providing any evidence, has been using an abused concept of national security... unjustifiably suppressing certain non-U.S. 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 U.S. laws.

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