U.S. Ambassador to Germany Richard Grenell warned the German government it could lose access to American intelligence if Germany insists on buying equipment from China’s Huawei telecommunications company for its 5G wireless network.

The Wall Street Journal reported Monday on the contents of Ambassador Grenell’s letter to the German government. In the letter, Grenell warned the Germans their access to U.S. intelligence could be scaled back if they use Huawei equipment, which the U.S. argues could be compromised by Chinese intelligence.

Grenell sent his letter the day after German Economy Minister Peter Altmaier said Germany wishes to avoid banning Huawei but will ensure all of the components in its 5G network are secure. Germany recently announced all electronics purchased for its mobile networks will be required to pass testing by the Federal Office for Information Security (BSI), and network operators will be strongly encouraged to buy equipment from a variety of suppliers to prevent any single company from developing a monopoly.

Grenell specifically addressed this point by warning that Chinese law requires all corporations to support Chinese spy agencies upon demand, so there is no way to permanently ensure the security of Huawei products by inspecting them at the time of purchase.

Perhaps anticipating China’s complaint that the Trump administration is cracking down on Chinese electronics to drive them off the market and create business opportunities for American firms, Grenell advised the Germans to hire Ericsson Communications of Sweden, Nokia of Finland, or Samsung of South Korea to build its 5G network.

The Wall Street Journal noted Grenell’s letter is the first time the U.S. government has expressly threatened repercussions for American allies that purchase Huawei equipment.

“An official from the German Economic Ministry dismissed the letter from Grenell, saying that it included no new information and reiterating that there was no evidence to support U.S. claims that Huawei posed a risk,” Asia Times reported on Monday.

The German official quoted by Asia Times also pointed to Huawei’s recently-filed lawsuit against the U.S. government as evidence that a ban against a single company might be legally difficult to enforce.