China is expanding its outposts in the South China Sea to include stationing for ships and potential airfields as part of its “aggressive” effort to exert sovereignty, the US intelligence chief said Thursday.

Director of National Intelligence James Clapper was speaking at a Senate Armed Services Committee hearing on worldwide threats. His comments underscore US concern over land reclamation activities that could fuel tensions between China and its neighbours over disputed islands and reefs.

“Although China is looking for stable ties with the United States it’s more willing to accept bilateral and regional tensions in pursuit of its interests, particularly on maritime sovereignty issues,” Clapper said.

He described China’s claims traced by a so-called nine-dash line a rough boundary covering more than 80 percent of the South China Sea as “exorbitant.”

The US is not a claimant of territory in the South China Sea but does claim a national interest in the peaceful resolution of the disputes in a region crucial for world trade.

China says its territorial claims have a historical basis and objects to what it consider US meddling.

Sen John McCain, the committee’s Republican chairman, displayed commercial satellite imagery showing expansion of the Chinese-occupied Gaven Reef in the Spratly Islands in the past year.

He said China’s expansion could allow it to employ weaponry, including anti-air and other capabilities.

Clapper said China was still in a construction phase so it was unclear what weaponry or forces it might deploy there.

He said such Chinese activities in the past year-and-a-half, combined with oil drilling near disputed islands that caused conflict with Vietnam, was a “worrying trend.”

The Centre for Strategic and International Studies last week said Vietnam, Malaysia and Taiwan have over the years modified existing land masses in the South China Sea, and the Philippines is planning to upgrade an airport and pier on an island it occupies.

But among the claimants, China is unusual in how it has been “dramatically changing the size and structure of physical land features,” the think tank said.

China has had a troop and supply garrison at Gaven Reef since 2003, but it began significant construction there last year, building a new artificial island, more than 18 acres in size. The main building on the new island appears to have an anti-aircraft tower, the center said.