"Too many jobs in China lost", Trump wrote.
The US Department of Commerce issued the trade ban following its determination of a violation of terms ZTE had agreed to after it was found selling embargoed technology to Iran in 2012.
The biggest blow to ZTE came last month, however, when the U.S. Department of Commerce, citing the company's failure to fulfill its plea deal and discipline the employees who sold components to Iran and North Korea, banned the sale of U.S. -made parts to the company for a period of seven years.
ZTE pleaded guilty a year ago to conspiring to violate US sanctions by illegally shipping USA goods and technology to Iran and entered into an agreement with the USA government. According to IDC data, ZTE sources more than 40 percent of its components from the US, creating a multibillion-dollar revenue stream for suppliers like Qualcomm and Intel. In a later tweet, Trump suggested the USA and China were moving forward on the trade negotiations but it was not clear how the ZTE case would fit into the bigger picture.
Trump's reassurance comes as the USA and China - the world's two largest economies - have hit each other with tariffs in an escalating trade battle. "But be cool, it will all work out!"
ZTE's fibre-optic networks depend on USA components and its cheap smartphones sold en masse overseas are powered by United States chips and the Android operating system.
Commerce Secretary Wilbur Ross last month accused ZTE of misleading the Department of Commerce and warned, "This egregious behavior can not be ignored".
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Mr Trump's reversal will likely have a significant impact on ZTE's U.S. suppliers, such as Qualcomm and Intel.
The US and China have also been posturing over import tariffs on particular goods, a move that could ignite a trade war.
In April, he said: "For many years, no president wanted to go against China economically, and we're going to do it". It's also the fourth largest provider of smartphones in the United States.
Experts said Trump's policy reversal was unprecedented. The President now seemingly has the upper hand at the negotiating table with the Chinese government; the U.S. can now effectively decide the future of the business.
Which brings us back to Trump.
California Representative Adam Schiff said: "Our intelligence agencies have warned that ZTE technology and phones pose a major cyber security threat".
Shares of Acacia, which got 30 percent of its total revenue in 2017 from ZTE, hit a record low after the ban was announced.