According to the Financial Times (FT), U.S. National Security Advisor Robert O’Brian has warned the UK about the dangers of using Huawei for its 5G networks.

In the ongoing war between the U.S. and Huawei, the U.S. has been pressuring allies to avoid the telecommunications company over allegations it provides a way for Beijing to spy on other countries. All Chinese corporations are required by law to assist the government when requested. Huawei, however, has closer ties with the Chinese government than many corporations. A number of wireless companies have already said they are using another manufacturer to build out their networks.

Now, O’Brian is ramping up the pressure on the UK as Prime Minister Boris Johnson weighs whether or not to ban Huawei as the U.S. has done. British officials had previously decided Huawei could be used with minimal risk as long as its equipment was kept from the “core” network and other sensitive applications, a stand the U.S. disagrees with.

“They are just going to steal wholesale state secrets, whether they are the UK’s nuclear secrets or secrets from MI6 or MI5,” O’Brien told FT. “It is somewhat shocking to us that folks in the UK would look at Huawei as some sort of a commercial decision. 5G is a national security decision.”