Donald Trump has declared a "national emergency" as his administration imposed severe sanctions on Chinese telecoms giant Huawei.
The president signed an executive order which – while it did not name Huawei – bars US companies from using telecommunications equipment made by firms deemed to pose a national security risk.
Hours later, the administration took further separate action as the commerce department added Huawei and 70 affiliated companies to a blacklist banning it from acquiring components and technology from US firms without government approval.
Commerce secretary Wilbur Ross said Mr Trump backed the decision that will "prevent American technology from being used by foreign-owned entities in ways that potentially undermine US national security or foreign policy interests".
Kevin Wolf, a former trade official in the Obama administration, said Huawei would be the largest business ever subjected to such controls.
He said they would have "ripple effects through the entire global telecommunications network".
"Huawei affiliates all over the planet depend on US content to function and if they can't get the widget or part of the software update to keep functioning then those systems go down," Mr Wolf said.
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US officials have previously labelled Huawei a threat and lobbied allies not to use its equipment in the development of new 5G mobile networks.
Huawei, which denies its products pose a security threat, said it was "ready and willing to engage with the US government and come up with effective measures to ensure product security".
A Chinese government spokesman said national security "should not be abused, and that it should not be used as a tool for trade protectionism".
He added: "China will take all the necessary measures to resolutely safeguard the legitimate rights of Chinese firms."
Huawei has attempted to calm security fears over its technology by offering to sign "non-spying" agreements – including with the UK government.
The company's executive vice president Jeremy Thompson this week told Sky's Ian King thaRead More – Source