The U.S. has been openly skeptical of Huawei's independence since at least 2012, when the House Intelligence Committee issued a report criticizing the tech company over its ties to China's communist government (and possibly the military) as well as a lack of transparency.
U.S. ambassador Richard Grenell said the USA would not be able to keep the same level of co-operation with German security agencies if Germany allowed Huawei or other Chinese firms to participate in its next-generation 5G mobile network, the Wall Street Journal reported.
Germany announced March 7 that it wouldn't ban any company from bidding on 5G contracts.
Europe's security agencies relied heavily on U.S. intelligence to stop terrorist attacks, the Journal noted.
There are only a few companies in the world that can provide the necessary infrastructure for the roll out of 5G.
Altmaier apparently received the US letter on Friday - one day after he said Germany doesn't want to put an outright ban on Huawei.
The US has been lobbying its allies to boycott Huawei due to national security risks.
"The Americans will assume that everything we share with Germany will end up with the Chinese", a senior State Department official told WSJ. Instead, he said, the country plans to "change its laws to guarantee all components used in the 5G networks are secure", as Deutsche Welle reports. "The 5G networks our allies buy won't be the networks that they eventually operate, as the software could be changed on a moment-to-moment basis by the manufacturer".
Huawei has in recent months met with over 200 politicians to try and smooth over security fears, he added.
Germany's telecommunication regulator has updated the requirements to bid for its 5G 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.