President Donald Trump has signed new laws in the United States targeting TikTok and WeChat, two of China's biggest apps.
Under the special executive orders, US businesses must stop working with the companies within 45 days.
The video-sharing app TikTok is one of the fastest-growing social platforms in the world, and has been downloaded more than two billion times.
The announcement comes as Microsoft - an American company - is in talks to buy TikTok ahead of a 15 September deadline set by Mr Trump.
TikTok said it was "shocked" by the president's order and is threatening to take legal action against the US.
TikTok is used by 80 million Americans every month, and the US is also home to its biggest star, Charli D'Amelio, who has over 75 million followers.
But some, including the president, have expressed concerns about the app, which is owned by Chinese firm ByteDance.
In both orders signed on Thursday, Mr Trump says "the spread in the United States of mobile applications developed and owned by companies in the People's Republic of China (China) continues to threaten the national security, foreign policy, and economy of the United States".
He refers to both apps as a "threat".
US officials and politicians have raised concerns that users' personal data, collected by ByteDance via TikTok, may end up being passed to the Chinese government.
However, TikTok has denied accusations that it is controlled by or shares data with it, and says all US user data is stored in the US, with a backup in Singapore.
"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," a TikTok spokesperson said.
It has also told users and regulators it would be "transparent" and let them see what data it is collecting and how it is collected.
"We are not political, we do not accept political advertising and have no agenda - our only objective is to remain a vibrant, dynamic platform for everyone to enjoy," the CEO of TikTok, Kevin Mayer, said.
"TikTok has become the latest target, but we are not the enemy," he added.
It comes after it was revealed in June that more than 50 apps, including TikTok, were found to be recording the text that users have been copying and pasting on Apple iPhones and other devices.
It is also at a time of heightened tensions between Donald Trump's government and the Chinese government over a number of issues, including how the coronavirus outbreak was handled in China.
There are a number of countries expressing concerns about the app.
TikTok was recently been banned in India, along with 58 other apps, with the Indian government claiming it is a threat to the country's national security. Australia is also considering banning TikTok.
Microsoft is in talks with TikTok's parent company ByteDance and the technology giant could end up buying TikTok's US business. TikTok's Canadian, Australian, and New Zealand operations are also on the cards.
President Trump has said America should be given a "substantial portion" of the purchase price if a deal does go ahead "because we're making it possible".
"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.
China has criticised Trump following his latest TikTok-related demands. It was stated in the country'sChina Daily newspaper - which the government runs - that Beijing would not accept the "theft" of a Chinese technology company.
A spokesperson for the Chinese foreign ministry has also accused the US president of "bullying" and "political manipulation".
WeChat is huge in China. It is sometimes described as being a social network, but it's really so much more - offering ways to make payments and get the news, in addition to messaging and other social activities.
It is perhaps best thought of as being a kind of secondary operating system that sits on top of iOS or Android.
It is also viewed as being a key part of China's system of spying on people in the country - requiring local users who have been accused of spreading negative rumours about the government to register a facial scan and voice print.
But in addition, it is allegedly commonly used by the Chinese Communist Party to pump out information promoting its political causes and interests to users.