The U.S. has been pushing back against China's technological expansion by asking that other countries refuse to adopt Huawei's 5G infrastructure, which Washington says poses security threats.
US intelligence officials have been issuing warnings for years that Huawei technology could possibly be co-opted by the Chinese government for espionage purposes, though they have never publicly released hard evidence and Huawei has repeatedly issued denials.
Last week, it filed a lawsuit against the USA government over a ban that restricts federal agencies from using its products, arguing it is "unconstitutional".
Among other things, European security agencies have relied heavily on USA intelligence in the fight against terrorism.
Germany announced on March 7 that it wouldn't ban any company from bidding on 5G contracts. Further, the US likely needs its allies to support the pushback against Huawei in order for the isolation tactic to have any meaningful effect.
Altmaier apparently received the US letter on Friday - one day after he said Germany doesn't want to put an outright ban on Huawei. However, they stressed that the U.S. would no longer trust Germany's lines because 'the Americans will assume that all information given to Germany will end up in China'.
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"Because 5G networks are largely software-defined, updates pushed to the network by the manufacturer can radically change how they operate", Marquis told CNN.
No response has since been released regarding the extent of what covers the intelligence reports that will be denied nor whether the same warning was also given to other countries.
The US is trying to recruit Germany in its campaign against Huawei. Recently, Germans used USA intelligence to stop the bombing that contains warfare agent ricin in Cologne previous year.
However, Germany's Federal Network Agency published tougher security guidelines for telecom providers, which require suppliers of 5G networks to be "trustworthy". However, Germany does not appear swayed by the US' arguments against Huawei, and plans to allow Huawei to bid for the project.
Grenell said in his letter 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.