China has installed equipment on two fortified outposts on the Spratly Islands capable of jamming communications and radar systems, which U.S. officials say signifies a stepped-up militarization of the South China Sea.

“China has deployed military jamming equipment to its Spratly Island outposts,” a Department of Defense official said.

The Chinese military is conducting a large exercise in the South China Sea, including maneuvers with China’s first aircraft carrier and air force and ground units. Some U.S. officials describe it as the largest military exercise in that region to date, according to the Wall Street Journal .

The move to install the communication and radar jamming systems will strengthen China’s ability to assert territorial claims in the South China Sea and stop U.S. military operations in that region, which includes some of the world’s largest and busiest shipping routes.

The assessment by the U.S. is supported by photo evidence by commercial satellites last month that show a suspected jammer system with its antenna extended on one of the seven Spratly outposts where China has built artificial islands.

Although China says the islands are for defensive purposes only, the activity has spurred fears that the outposts could be used to enforce territorial claims overlapping with Brunei, Malaysia, Taiwan, Vietnam and the Philippines.

China’s Defense Ministry has not yet responded to a request for comment.