Ignoring the warning may result in the United States downgrading its intelligence-sharing cooperation with Germany, the letter dated last Friday said.
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The U.S. has warned Germany that if Chinese tech companies such as Huawei help build its new 5G telecom infrastructure, U.S. agencies might not share as much intelligence with the German government as they now do. -German cooperation could only continue at its current level if Chinese firms were excluded from the construction of the network.
Germany's potential use of Huawei in its 5G rollout is a sticking point for the US.
The battle with Huawei is one of many conflicts in Washington's disputes with China, but it is also exacerbating tensions in US-German ties that are strained over the Nord Stream 2 gas pipeline, the 2015 Iran nuclear deal and trade.
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In his letter, Grenell said that Chinese companies, under Chinese law, can be required to support China's security agencies and that inspections of Huawei software could not ensure there were no vulnerabilities, the newspaper said.
Among other things, European security agencies have relied heavily on USA intelligence in the fight against terrorism. As CNBC reports, both the 2017 National Intelligence Law and the 2014 Counter-Espionage Law mandate that Chinese firms comply with state security efforts.
The U.S. has warned Germany about possible repercussions of using Huawei in its upcoming 5G mobile network, citing security concerns.
German minister Altmaier confirmed he had received the letter, but told AFP he could not comment on its contents, adding: "We will respond quickly".
The year ended with the arrest of Huawei's chief financial officer in Canada at USA request, to the consternation of China.