ChinaPower, a unit of the Center for Strategic and International Studies, published photos of a large vessel under construction at the Jiangnan Shipyard in Shanghai.

The bow and hull are already under assembly, the pictures indicated, and are likely to be the beginnings of the 80-85,000 tonne type 002 aircraft carrier the Chinese navy has been known to be planning.

"Visible through the clouds and mist is what looks to be a bow and main hull section of a large vessel," ChinaPower said.

"While details regarding the Type 002 are limited, what is observable at Jiangnan is consistent with what is expected for the People's Liberation Army Navy's third aircraft carrier."

China's first carrier, the three decade-old, 66,000 tonne Liaoning, was acquired from Russia.

The second, a home-built carrier designated Type 001A and based on the Liaoning's design began sea trials, one year ago.

ChinaPower said that, based on the photos, the Jiangnan shipyard appears to be building a new tower crane, a floodable ship basin and a launching channel to accommodate the huge new carrier.

In a report published online, ChinaPower said the carrier is expected to be completed in 2022.

In January a senior Chinese naval expert said the country needs "at least" three carriers to defend its coastline and global interests.

"Our country has an 18,000-kilometre (11,000-mile) long coastline. Also, our economy is outward looking and our interests abroad are growing," said Commodore Zhang Junshe, a member of the Naval Research Institute.