China has started deploying communications and radar jamming equipment in two of its outposts in the Spratly archipelago in the South China Sea, US defense officials said.

First reported by the Wall Street Journal on Monday, the US assessment is backed by a satellite photo taken by commercial satellite company DigitalGlobe last month.

“It shows a suspected jammer system with its antenna extended on Mischief Reef (Panganiban Reef), one of seven Spratly outcrops where China has built fortified artificial islands since 2014, moving sand onto rocks and reefs and paving them over with concrete,” the report said

Mischief Reef, the largest of China’s seven outposts in the Spratlys, is located within the 200-nautical mile exclusive economic zone of the Philippines. A tribunal in The Hague announced in 2016 ruled that this reef belongs entirely to the Philippines and it has sovereign rights over the area.

The jamming equipment was deployed in the past 90 days on Fiery Cross (Kagitingan Reef) and Mischief Reef, the report said.

“While China has maintained that the construction of the islands is to ensure safety at sea, navigation assistance, search and rescue, fisheries protection and other nonmilitary functions, electronic-jamming equipment is only for military use,” the U.S. Defense Department official told the Wall Street Journal.

Sought for comment, Philippine defense and military establishments have not yet responded as of posting time.

China has developed and slowly militarized its artificial islands in the Spratlys in recent years, saying it’s only for defensive purposes. Three of the seven outposts — Mischief Reef, Fiery Cross Reef and Subi Reef — have 3-kilometer runways, hangars, ports, missile shelters, among other facilities.

The report comes after China held off Hainan Island its largest naval exercises in the disputed waters to date, described by analysts as “unusually large display of the Chinese military’s growing naval might.”