HONG KONG (BLOOMBERG) - China's South China Sea island-building has expanded into the Paracel Islands, according to satellite photos published as South-east Asian leaders gather in California for two days of talks with US President Barack Obama.

Images posted on The Diplomat website show dredging and filling at two new sites in the Chinese-held island chain about 15km from a Chinese military base on Woody Island.

The photos show a helicopter base under construction, according to the article by writer Victor Robert Lee, an analyst who tracks and analyses China's land reclamation efforts using satellite imagery.

The Paracels, which are also claimed by Vietnam and Taiwan, sit north-west of the Spratlys chain, where China's island- building programme has prompted protests from the US and countries with overlapping claims.

The new evidence of reclamation came as Mr Obama told leaders of the Association of South-east Asian Nations (Asean) that the US would seek to build a unified approach to ensure maritime security.

"Here at this summit, we can advance our shared vision of a regional order where international rules and norms, including freedom of navigation, are upheld and where disputes are resolved through peaceful, legal means," Mr Obama said on Monday (Feb 15) in his welcoming address at the Sunnylands estate in California.

China claims more than 80 per cent of the South China Sea and has over the past two years built 1,214ha of land in the Spratlys including a runway and other facilities that could be used for military purposes.

The claims clash with those of four Asean members, including Vietnam and the Philippines, over a body of water that hosts US$5 trillion (S$7 trillion) a year in global trade.

US officials have repeatedly requested China and others to stop reclaiming land and halt the militarisation of the area. China's Foreign Minister Wang Yi told Asean foreign ministers in August that China had completed its land reclamation and was now focusing on building civilian support facilities there that it would share with other nations.

The latest dredging began sometime after Dec 2, and was creating terrain on a reef adjacent to North Island, which has been occupied by China since 1950, according to Mr Lee's article.

At China-occupied Tree island - 5.5km north-west of North Island - a dredger can be seen expanding a port area and piping sediment onto a new area of fill. Satellite images show the work at Tree Island started after Oct 18.

Last month, the US sent the USS Curtis Wilbur into disputed waters near the Paracels to contest the "excessive" maritime claims of China, Taiwan and Vietnam.

It was the second time in less than six months a US naval ship has challenged China with a freedom-of-navigation voyage.