China's South China Sea island-building has expanded into the Paracel Islands, according to satellite photos published as Southeast Asian leaders gathered in California for two days of talks with U.S. President Barack Obama.

Images posted on The Diplomat website show dredging and filling at two new sites in the Chinese-held island chain about 15 km (9 miles) from a Chinese military base on Woody Island. The photos show a helicopter base under construction, according to the article by 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 northwest of the Spratlys chain, where China's island- building program has prompted protests from the U.S. and countries with overlapping claims. The new evidence of reclamation came as Obama told leaders of the Association of Southeast Asian Nations (ASEAN) that the U.S. would seek to build a unified approach to ensure maritime security.