China may be preparing to mobilise "hundreds" of its surface-to-air missiles currently stationed in Hainan Island.
US military officials said the missiles, which Beijing recently shipped to its non-contested Hainan Island in the South China Sea, will be moved to the country's disputed man-made islands over coming months.
The officials told Fox News Hainan Island would likely serve as a training site before the missiles are deployed early next year to the disputed Spratly Islands or Woody Islands.
The equipment includes a number of short, medium and long-range weapons. One of these, a military unit of the advanced SA-21 system, would be capable of knocking out aircraft from more than 400km away.
The officials also said the total number of surface-to-air missiles on Hainan could reach 500.
If true, this puts China's actions in the South China Sea at odds with previous statements made by President Xi Jinping.
While strengthening its presence in the South China Sea is a long-term goal for Beijing, Xi pledged not to "militarise" the disputed region during a visit to the US last year.
"This is another example of the adventurous and aggressiveness of the Chinese in the face of an anaemic and feckless set of policies that we've seen over the last eight years," said retired air force Lieutenant General David Deptula, the former head of air force intelligence.
This latest development comes days after Beijing returned an unclassified underwater research drone in the South China Sea.
The Pentagon accused China of stealing the drone, making global headlines after Donald Trump slammed the Chinese government on Twitter, and then told them to "keep it".
Earlier this month, the Asia Maritime Transparency Initiative think tank (AMTI) reported that China appeared to have installed weapons on all seven of its artificial islands in the region.
AMTI said it had been tracking construction of hexagonal structures on Fiery Cross, Mischief and Subi reefs in the Spratly Islands since June and July. China has already built military length airstrips on these islands.
A fresh batch of satellite photos taken in November show these are being completed as point-defence fortifications housing radar-guided anti-aircraft and antimissile guns.
China has said military construction on the islands will be limited to necessary defensive requirements.