Huawei Technologies, the Chinese company put on a US blacklist because of national security concerns, secretly helped North Korea build and maintain its commercial wireless network, the Washington Post reported on Monday, citing sources and internal documents.

The Chinese telecommunications company partnered with a state-owned Chinese firm, Panda International Information Technology, on a number of projects in North Korea over at least eight years, the Post reported.

Such a move would raise questions about whether Huawei, which has used US technology in its components, violated American export controls to furnish North Korea with equipment, according to the Post.

The US put Huawei on a blacklist in May, citing national security concerns. The move banned US companies from selling most parts and components to Huawei without special licences, but President Donald Trump said last month that US firms could resume sales, in a bid to restart trade talks with Beijing.

Huawei did not immediately respond to a request for comment, but said in a statement to the Post it had “no business presence” in North Korea. It was not immediately possible to reach the Panda Group.

The US Department of Commerce, which also did not immediately respond to a request for comment, has investigated possible links between Huawei and North Korea since 2016 but has not publicly connected the two, the Post said.

Huawei and Panda vacated their Pyongyang office in the first half of 2016, the newspaper reported.