WASHINGTON (AFP) - China's efforts to reclaim land in the South China Sea and build up its navy are increasingly spurring regional neighbours to oppose Beijing, US Defence Secretary Ashton Carter said on Thursday.

"Chinese behaviour is having the effect of self-isolation, and it's also galvanising others to take action against it," Carter told lawmakers on the House Appropriations Committee.

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.

Carter said other nations in the region are responding by stepping up their own maritime defence activities and aligning themselves with the United States.

"Old allies, like Japan, South Korea, Australia and the Philippines, and then new partners, like Vietnam and India, that are working with us increasingly," he said.

Carter's comments came shortly before Admiral Harry Harris, the commander of the US Pacific Command, was due to brief Pentagon reporters about China's naval activities.

Harris warned lawmakers this week that China is "clearly militarising" the South China Sea and said the United States would increase its "freedom of navigation" operations - in which a US warship sails within 12 nautical miles of islets claimed by China - as a way of rebutting its assertions of sovereignty.

Satellite imagery released 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.