https://www.bbc.com/news/world-us-canada-48289550
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 does 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.
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"This Executive Order addresses the threat posed by foreign adversaries to the nation's information and communications technology and services supply chain," says @SecretaryRoss in statement, praising @POTUS for declaring this national emergency.
1:45 PM - 15 May 2019 from Washington, DC
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Subject: Trump declares National Emergency over IT threats 
Wed May 15, 2019 5:25 pm
