A PLA Navy fleet including the aircraft carrier Liaoning, submarines, vessels and fighter jets take part in a review in the South China Sea on April 12, 2018.

WASHINGTON — China has quietly installed anti-ship cruise missiles and surface-to-air missile systems on three of its fortified outposts west of the Philippines in the South China Sea, a move that allows Beijing to further project its power in the hotly disputed waters, according to sources with direct knowledge of U.S. intelligence reports.

Intelligence assessments say the missile platforms were moved to the outposts in the Spratly Islands within the past 30 days, according to sources who spoke on the condition of anonymity.

The placement of the defensive weapons also comes on the heels of China's recent South China Sea installation of military jamming equipment, which disrupts communications and radar systems. By all accounts, the new coastal defense systems represent a significant addition to Beijing's military portfolio in one of the most contested regions in the world.

The United States has remained neutral – but expressed concern – about the overlapping sovereignty claims to the Spratlys.

"We have consistently called on China, as well as other claimants, to refrain from further land reclamation, construction of new facilities, and militarization of disputed features, and to commit to managing and resolving disputes peacefully with other claimants," a Pentagon official told CNBC when asked about China's recent military activity in the area. "The further militarization of outposts will only serve to raise tensions and create greater distrust among claimants."

The recent intelligence, according to sources, indicates the deployment of anti-ship cruise missiles and surface-to-air missiles on Fiery Cross Reef, Subi Reef and Mischief Reef in the Spratly Islands. The Spratlys, to which six countries lay claim, are located approximately two-thirds of the way east from southern Vietnam to the southern Philippines.

The land-based anti-ship cruise missiles, designated as YJ-12B, allow China to strike surface vessels within 295 nautical miles of the reefs. Meanwhile, the long-range surface-to-air missiles designated as HQ-9B, have an expected range of targeting aircraft, drones and cruise missiles within 160 nautical miles.

The defensive weapons have also appeared in satellite images of Woody Island, China's military headquarters in the nearby Paracel Islands.

"Woody Island serves as the administrative and military center of China's presence in the South China Sea," Gregory Poling, Center for Strategic and International Studies fellow and director of the Asia Maritime Transparency Initiative, told CNBC in a prior interview.

"We assume that anything we see at Woody will eventually find its way farther south to more directly menace China's neighbors," he added.