China on Tuesday expressed strong indignation over the illegal intrusion by the United States into the islands and reefs of the Nansha Islands.

Chinese foreign ministry spokesman Lu Kang said at a daily briefing that the US actions posed a serious threat to China's national sovereignty and security interests.

A US missile destroyer Tuesday entered the China's territorial waters in the South China Sea without asking permission from the Chinese government.

Lu also said that China had indisputable sovereignty over the Nansha Islands and its surrounding waters.

He added that China has always fulfilled its obligations to maintain the freedom of navigation and flight over the region. The spokesman said there had never been any problem with the freedom of navigation and flight there.

"The United States had come out with the so-called 'Freedom of Navigation' scheme before the United Nations Convention on the Law of the Sea was signed in 1979. This is an obvious military challenge to the new marine order," said Lu.

"It met with opposition from many countries and the United States has up to now refused to join the convention. It has actually placed its own interests above this convention."

"By flaunting the banner of freedom of navigation and flight, the United States has flexed its military muscles by sending warships and military planes close to and even into the related islands and reefs and their surrounding sea and air space for provocation."

"This is the biggest threat to the peace and stability and the freedom of navigation and flight on the South China Sea."