China has sent fighter jets to a disputed island in the South China Sea, where it deployed surface-to-air missiles earlier this month, U.S. government sources told Reuters on Tuesday.

One U.S. official, however, noted that China regularly sent jets to Woody Island, part of the Paracel archipelago controlled by Beijing. The deployment was first reported by Fox News.

This follows several "militarization" actions by China in the region, which it claims control of, in competition with several other Asian nations including Vietnam and the Philippines. The Philippines last October won the right to have its territorial claims over the disputed area heard by an arbitration court in the Hague, infuriating China.

Control of the region is valuable because more than $5 trillion worth of global trade passes through the South China Sea each year, and China has been accused of ramping up tensions over control in recent years by building artificial islands on reefs, on which it has added airstrips and other military-style installations.

The U.S. regularly exercises its right to "freedom of navigation" by sending naval vessels through the South China Sea, including within 12 nautical miles of islands claimed by China. Beijing has responded by calling these actions "provocations" that risked a military response.

A U.S. think tank reported on Monday that China may be installing a high-frequency radar system on the Cuarteron Reef in the Spratly Islands that could significantly boost its ability to control the strategic sea.

And last Thursday, the U.S. accused China of inflaming the issue by its apparent deployment of surface-to-air missiles on Woody Island.