U.S. President Donald Trump has declared a national emergency to protect US computer networks from foreign adversaries.
The president signed an executive order effectively barring US companies from using foreign telecoms believed to pose a security risk to the country.
Mr Trump did not name any company specifically in the order.
However, analysts suggest it is mainly directed at Chinese telecommunications giant Huawei.
A number of countries have raised concerns the company's products could be used by China for surveillance.
The Chinese company - which is the world's largest maker of telecoms equipment - has denied that its work poses any risks of espionage or sabotage.