The US Navy plans to increase "freedom of navigation" operations in the South China Sea as Beijing continues its military buildup in the contested waterway, a US admiral said Wednesday.

The sailings involve a US warship coming within 12 nautical miles of islets claimed by China as a way of rebutting Beijing's assertions of sovereignty.

Since October, the Navy has carried out two such freedom of navigation operations in the South China Sea, saying the missions are an important way of upholding international law.

"We'll be doing them more and we'll be doing them with greater complexity in the future," Admiral Harry Harris, the commander of the US Pacific Command, told lawmakers in Washington.

"We'll fly, sail and operate wherever international law allows."

China's claims to almost all of the South China Sea are widely disputed and the body of water has long been viewed as a potential flashpoint.

China is using dredgers to turn reefs and low-lying features into larger land masses for runways and other military uses to bolster its claims of sovereignty in the region.

Satellite imagery released by a Washington think tank this week shows China is installing radar facilities on its artificial islands.

China has also deployed surface-to-air missiles and lengthened a runway to accommodate fighter jets on one such islet, Woody Island.

"China's intent to militarize the South China Sea is as certain as a traffic jam in DC," Harris told the House Armed Services Committee in reference to congestion on the streets of Washington.

He did not offer specifics on the upcoming freedom of navigation sailings but suggested they be carried out by a type of destroyer that "is well able to defend itself should those operations go awry."