Construction work by China on some garrisoned islands and reefs on its Nansha Islands was defended by a senior Chinese diplomat on Tuesday.

Such work is proceeding "at a pace and on a scale befitting its (China's) international responsibilities and obligations", the diplomat said.

Ouyang Yujing, director-general of the Chinese Foreign Ministry's Department of Boundary and Ocean Affairs, was responding to accusations that China's construction activities on some islands and reefs are outpacing and "outsizing" those by other countries in the South China Sea.

"China is a big country that shoulders more international responsibilities and obligations," he said.

These responsibilities include "search and rescue, disaster prevention and mitigation, meteorological observation, ecological conservation, navigation safety and fishery services", he said.

The work is designed to "provide better services for the ships of China, its neighbors and other countries whose ships and boats pass through the South China Sea", he said.

US officials, including Assistant Secretary of State for East Asian and Pacific Affairs Daniel Russel, have claimed recently that equitable resolution of disputes and freedom of navigation are being "challenged" by Chinese activities in the South China Sea.

Ouyang said freedom of navigation and overflights in the region have never been affected by such disputes.

Construction and maintenance of facilities on China's garrisoned islands and reefs "will not undermine countries' freedom of navigation", he said.

"On the contrary, it will facilitate joint response to challenges on the sea and provide for increased safety of navigation," he said.

China has accused the Philippines and some other countries of illegally occupying some islands and reefs in the Nansha Islands.

Ouyang stressed that China's construction activities and those of other countries are "totally different in nature", and again urged the other nations to immediately withdraw their personnel and facilities from the islands and reefs they have occupied illegally.

Speculation has arisen over whether China will establish an Air Defense Identification Zone in the South China Sea once the construction is completed.

Ouyang said China has the right to establish an ADIZ, and this has nothing to do with territorial or maritime disputes.

He also dismissed speculation that Chinese construction on the islands and reefs is aimed at bolstering the legal status of the Nansha Islands.

China's sovereignty and relevant claims to rights in the South China Sea have been formed in "the long course of history" and upheld by successive Chinese governments, he said.

"This position has adequate historical and legal basis. There is no need to have it strengthened through construction activities on the islands and reefs."