China has reclaimed more land in the disputed Spratly Islands of the South China Sea than previously known, according to a new Pentagon report, and a senior U.S. defence official said it was unclear whether Beijing had stopped island-building in the region.

"China has said that it ... has stopped reclamation. ... It's not clear to us that they've stopped," Assistant Defence Secretary David Shear told a Pentagon briefing on Friday as the department released a report on its Asia-Pacific Maritime Security Strategy.

Shear said ongoing Chinese activity may simply be "finishing up" what Beijing started rather than adding more territory, but "we are watching it carefully" for signs of further construction or militarization.

The report said Beijing was in the process of completing a runway on one of its seven man-made outposts. Once the airstrip on Fiery Cross Reef is operational, China could potentially use it as an alternative runway for carrier-based planes, allowing the Chinese military to conduct "sustained operations" with aircraft carriers in the area, the report said.

China's sole aircraft carrier, a Soviet-era ship bought from Ukraine and refitted in China, has carried out exercises in the South China Sea but is not yet fully operational. Some experts believe China will deploy domestically built carriers by 2020 as part of plans to develop an ocean-going "blue water" navy.

At the reclamation sites in the Spratlys where China is in the building phase, it has excavated deep channels and constructed new berthing areas to allow access for larger ships, the report said.

"The infrastructure China appears to be building would enable it to establish a more robust power projection presence into the South China Sea," it added.

Since China's land reclamation efforts began in December 2013, it had reclaimed more than 2,900 acres (1,170 hectares) of land as of June 2015, the report said. U.S. officials had previously put the total at 2,000 acres (809 hectares).

In a statement, China's Foreign Ministry said China had "completed the relevant island and reef area reclamation project" at the end of June.

Construction activities were "completely within the scope of China's sovereignty", it added.

In early August, Foreign Minister Wang Yi said Beijing had halted land reclamation.

China says the outposts will have undefined military purposes, as well as help with maritime search and rescue, disaster relief and navigation.

Asked about the possibility of China declaring an air defence identification zone, or ADIZ, over the South China Sea, Shear said he thought the United States would reject it, just as it did the ADIZ that Beijing declared over the East China Sea.

An ADIZ requires an aircraft flying into the zone to identify itself, giving the country that established the zone more time to protect itself against potentially hostile aircraft.

China claims most of the South China Sea, through which $5 trillion in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei also have overlapping claims.

The reclamation campaign significantly outweighed efforts by other claimants in size, pace and nature, the Pentagon report said.

China had reclaimed 17 times more land in 20 months than the other claimants combined over the past 40 years, accounting for approximately 95 percent of all reclaimed land in the Spratlys, it added.