As the US and China plunge into a trade war, Beijing is apparently preparing for a different type of warfare that some fear may be on the horizon.

Following news earlier this year that China installed military jamming equipment on contested islands in the South China Sea, CNBC reported on Thursday they have begun testing it out.

China is “quietly testing” the electronic warfare assets, the report said, citing sources who have seen US intelligence reports. According to the assessments, this is the first known use of the equipment.

In April, The Wall Street Journal quoted a US Defense Department official as saying that “China has deployed military jamming equipment to its Spratley Island Outposts,” as part of what the US describes as an aggressive military buildup in the disputed territory.

In addition to the electronic equipment, surface-to-air and anti-ship cruise missiles have also been deployed on three outposts in the South China Sea.

China’s claims of sovereignty over all of the islands located in the South China Sea overlap with competing claims from other nations in the region, including Malaysia, the Philippines and Vietnam.

“China has built a massive infrastructure specifically—and solely—to support advanced military capabilities that can deploy to the bases on short notice,” Admiral Harry Harris, then-commander of the US Pacific Command, told the Senate Armed Services Committee in March.

Beijing contends that its buildup is defensive in nature and has accused Washington of destabilizing the region with naval exercises, including so-called freedom of navigation operations.