The US has expressed concern at China's progress in maritime zones it shares with its neighbours but Beijing has said its activities in its own territory were legitimate and justified.
Media reports quoted the US intelligence chief James Clapper as saying Thursday that China was expanding its outposts in the South China Sea to include stations for ships and potential airfields as part of its "aggressive" effort to exert sovereignty.
"China has shown restraint and a responsible attitude in the South China Sea. Outsiders have no right to make groundless accusations," state-run Xinhua news agency quoted foreign ministry spokesperson Hong Lei as saying at a daily news briefing Friday.
He urged the US to honour its commitments, to be prudent with its words and actions, and do more to contribute to China-US relations and regional peace and stability.
At a hearing of the Senate Armed Services Committee Thursday, Clapper said that China was adopting a very aggressive role in the South China Sea with the construction of artificial islands on previously uninhabited atolls, pointing to a worrying trend.
According to Clapper, examples such as the transformation of the Hughes Reef, in the disputed Spratly Islands (called Nansha in Chinese), demonstrates China's intention of consolidating its maritime expansion with the construction of airfields, Efe news agency reported.
Hughes Reef, which a year ago was still under water, is now a construction zone with a helipad and a dredged area to allow the berthing of deep draft ships, according to images revealed last week by the IHS Janes study centre.
"What the Chinese are doing here is building airfields to launch aircraft for patrols and surveillance in an area considered under its sovereignty," said Clapper.
Clapper also said that the animosity that is motivating Chinese expansion among the majority of its neighbours like Vietnam, Philippines, Japan and Malaysia can be a good thing, because then it would help in deciding if joint action can be taken against Beijing.
Republican Senator John McCain, one of the "hawks" of the party, said that at the moment attention was focused elsewhere in the world but the actions of China were very significant.
Clapper also said that China's impressive military modernisation was deliberately designed to counter US strength.
"They want to deny our potential in monitoring, control and armament (in Asia-Pacific)," Clapper said in commenting on the rapid expansion and military investment by Beijing.
