The White House has warned of "consequences" for what it called militarization of the South China Sea, following reports that Beijing had installed missiles on three outposts in the hotly contested region.

White House spokeswoman Sarah Huckabee Sanders said Thursday “there will be near-term and long-term consequences” in response to the reported military buildup in the South China Sea.

Sanders said the Trump administration had “raised concerns” about the issue “directly” with Chinese authorities.

White House Press Secretary Sarah Huckabee Sanders speaks during the daily press briefing at the White House in Washington, DC, May 3, 2018. (Photo by AFP)

Citing sources with knowledge of US intelligence, CNBC News reported on Wednesday that China had installed anti-ship cruise missiles and surface-to-air missile systems on three of its fortified outposts in the South China Sea.

The Pentagon has also reacted to the intelligence assessment.

"We have been very vocal about our concerns about them militarizing these artificial islands," Defense Department spokeswoman Dana White said Thursday. "China has to realize that they have benefited from the free navigation of the sea and the US Navy has been the guarantor of that."

The reports came on the heels of China's military drills in the South China Sea which it described as being part of routine military training.

Intelligence assessments say the missile systems were deployed to Fiery Cross Reef, Subi Reef and Mischief Reef in the Spratly Islands over the past month.

Officials said the deployment would allow Beijing to further project power in the region.

Chinese Foreign Ministry spokeswoman Hua Chunying defended the country’s peaceful construction in the islands, including "the deployment of necessary national defense facilities."

"The relevant deployment targets no one. Anyone with no invasive intention will find no reason to worry about this," the spokeswoman said in a press briefing. "We hope that the relevant party could view this matter in an objective and calm way."

China previously announced in 2016 it had deployed similar defense systems to Woody Island in the Paracel Islands, on the northwestern edge of the sea.

Sailors prepare FA-18 Hornet fighter jets for take off during a routine training aboard the US aircraft carrier Theodore Roosevelt in the South China sea on April 10, 2018. (Photo by AFP)

The US has stepped up its military presence in the South China Sea under the pretext of freedom of navigation operations in international waters.

Experts studying China cite these operations as a reason behind the deployment by China of defensive facilities in the region.

The South China Sea serves as a gateway to global sea routes, though which about $3.4 trillion of trade passes each year. Vietnam, Taiwan, the Philippines, Malaysia, and Brunei have overlapping claims with China to parts of the sea.