Chinese Foreign Ministry spokesman Hong Lei said the country's activities on shoals and in surrounding waters it claims are "reasonable, legitimate and legal" and that its attitude has been one of "restraint and responsibility."
China says it has historical claims to a huge swath of the South Sea China that overlaps the claims of several neighbours including Vietnam, Malaysia, Taiwan and the Philippines, and it objects to what it considers US meddling.
US Director of National Intelligence James Clapper cited China's expansion of its outposts, including for the stationing of ships and potential airfields, at a US Senate hearing in Washington yesterday.
His comments underscored US concerns about land reclamation activities that could fuel tensions between China and its neighbours.
Clapper described China's claim to more than 80 per cent of the South China Sea as "exorbitant."
Hong said China hopes the US can be more "circumspect" regarding the issue. "No other country has a right to make unfounded accusations," he said at a daily ministry briefing.
But the think tank said China is unusual in how it has been "dramatically changing the size and structure of physical land features."
China has had a troop and supply garrison at Gaven Reef since 2003, and began significant construction there last year, building a new artificial island of more than 18 acres (7 hectares).
The main building on the new island appears to have an anti-aircraft tower, the centre said.