"China has indisputable sovereignty over Nansha Islands," Chinese Foreign Ministry spokesperson Hua Chunying told media.
China calls the island chain as Nansha while Vietnam calls them Spratly Islands.
The statement came after a US military spokesman claimed the reclamation project was one of several pursued by China but the first that could accommodate an airstrip.
"The external forces have no right to make irresponsible remarks," she said when asked about US military's comments.
According to a report last week by IHS Jane's Defence, new details emerged of a land reclamation project by China on Fiery Cross Reef, Yongshu in Chinese, in the Spratly Islands.
China claims almost all of the South China Sea, while Brunei, Malaysia, the Philippines, Taiwan and Vietnam contests the Chinese claims on the resource-rich region.
"Which means that the countries directly concerned should be directly engaged in negotiations and consultations in resolving it," she said.
The Hong Kong-based South China Morning Post cited Jane's Defence journal report and said China had over the past three months used dredgers to build an island about 3,000 metres long and 200 to 300 metres wide on Fiery Cross Reef.
Days after the report, US military spokesman Lieutenant Colonel Jeffrey Pool called on China and other governments to cease such construction.
