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.

The South China Sea, which is home to more than 200 specks of land, serves as a gateway to global sea routes where approximately $3.4 trillion of trade passes annually.

The numerous overlapping sovereign claims to islands, reefs and rocks — many of which disappear under high tide — have turned the waters into an armed camp. Beijing holds the lion's share of these features with approximately 27 outposts peppered throughout.

Beijing's interest in developing the crumbs of land across the South China Sea is by no means new.

For instance, China first took possession of Fiery Cross Reef and Subi Reef in 1988 and has since outfitted the features with deep-water ports, aircraft hangars, communication facilities, administration offices and a 10,000-foot runway.

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^{Satellite images of Subi Reef from July 2012 and December 2017. Source: Center for Strategic and International Studies.}

Last week, U.S. Navy Adm. Philip Davidson, the expected nominee to replace U.S. Pacific Command Chief Adm. Harry Harris, described China's increased presence in the South China Sea as "a substantial challenge to U.S. military operations in this region."

In written testimony to the U.S. Senate Armed Services Committee, Davidson said the development of China's forward operating bases in the hotly contested waters appear to be complete.

"The only thing lacking are the deployed forces. Once occupied, China will be able to extend its influence thousands of miles to the south and project power deep into Oceania," Davidson wrote. "In short, China is now capable of controlling the South China Sea in all scenarios short of war with the United States."

Davidson's comments echo a steady drumbeat of warnings made by Harris regarding China's growing strength.

Earlier this year, Harris told Congress that Beijing's impressive military buildup, including its pursuit of hypersonic weapons, could challenge the United States "across almost every domain."

"While some view China's actions in the East and South China Seas as opportunistic, I do not. I view them as coordinated, methodical and strategic, using their military and economic power to erode the free and open international order," Harris told the House Armed Services Committee.

Harris, whom President Donald Trump is reportedly set to nominate as U.S. ambassador to South Korea, currently oversees approximately 375,000 military personnel and is responsible for defending a theater that spans nearly half of the Earth's surface.

"Ladies and gentlemen, China's intent is crystal clear. We ignore it at our peril," Harris said.

Beijing has upped the ante with its latest deployment of anti-ship cruise missiles and surface-to-air missile systems to fortified outposts in the hotly contested waters of the South China Sea.

And it means China will likely soon send warplanes to the area, too, according to an expert on the region.

"This should be seen as China crossing an important threshold. Missile platforms present a clear offensive threat," Gregory Poling, Center for Strategic and International Studies fellow and director of the Asia Maritime Transparency Initiative, told CNBC. "[The missile deployment] is a pretty clear threat to the other claimants and furthers China's goal of establishing complete control over the water and airspace of the South China Sea."

According to U.S. intelligence reports, the installations mark the first Chinese missile deployments to 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.

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

"It has been clear that these were coming since at least last year when China constructed shelters for them at each of the islands, but the actual deployments are still significant," Poling said. "Now every ship or plane moving near the Spratlys is operating within Chinese missile range."

While the installation of missile platforms are new to the Spratlys, China has already deployed similar systems in the nearby Paracel Islands.

Satellite images of Woody Island, Beijing's military headquarters in the South China Sea, show deployments of Y-8 transport aircraft as well as J-10 and J-11 fighter jets.

China first took possession of Woody Island in 1955 and has since outfitted it with ports, aircraft hangars, communication facilities, helipads and a runway.

Poling notes that this particular outpost serves as a blueprint for China's future developments on Fiery Cross Reef, Subi Reef and Mischief Reef in the Spratly Islands.

Looking ahead, Poling said China will most likely work to boost its air power on the reefs in the Spratlys.

"Next up will be deployments of combat aircraft, which we should expect sooner rather than later," he said.

In terms of airspace capabilities, China is the only nation to boast airstrips that are 10,000 feet long, which can accommodate cargo planes, surveillance aircraft, fighter jets and bombers.