China is building a major underground nuclear submarine base on the southern tip of Hainan Island, defence group Jane's said Friday.

Jane's Intelligence Review, a respected defence periodical, said satellite images of the base from imagery provider DigitalGlobe were the first confirmation of its existence.

Although Beijing is displaying no overt aggression, the base could mean an increase in its strategic capability in the South China Sea and considerably further afield, Jane's analysis said.

"Jane's can confirm that the satellite pictures show that China is constructing a major underground nuclear submarine base near Sanya, on Hainan Island off its southern coast," the group said.

The Daily Telegraph, which reported the satellite images, called the base a "vast, James Bond-style edifice capable of concealing up to 20 nuclear-powered submarines, which will enable China to project its power across the region."

The British broadsheet said in an editorial that it was a sign of China's secretive side and Beijing "too often seeks to conceal its activities and becomes defensive when questioned."

Janes said that Asian military sources had told it about the base in 2002, but the photographs provided independent verification.

The satellite images showed the harbour layout and a Type 094 nuclear-powered ballistic missile submarine at the base, said Jane's.

Others show three Luyang guided missile destroyers and a Jiangwei 2 guided missile frigate moored on a jetty, it said.

There are believed to be 11 tunnel openings at the base, it was reported, with each entrance, carved into the hill-side, stretching to a height of about 60 feet (18 metres). Pictures showed two of the tunnel entrances.

Another showed construction operations involving engineering and excavation barges.

The extent of construction indicates that the Sanya base could become a key future hub for the Chinese navy's aircraft carriers and other power-projection ships, Jane's said.

The Chinese navy moved its first Type 094 submarine to Sanya in December 2007, it added.

The identification of an underground submarine base and the positioning of China's most advanced sub-surface combatants at Sanya could have implications for China's control of the South China Sea and the strategically vital straits in the area, said Jane's.

"For both regional and extra-regional powers, it will be difficult to ignore that China is now building a major naval base at Sanya and may be preparing to house and protect a large proportion of its nuclear forces here and even operate them from this base," the group said.

"This development so close to the south-east Asian sea lanes so vital to the economies of Asia can only cause concern far beyond these straits."

Jane's Intelligence Review editor Christian Le Miere said: "China's nuclear and naval build-up at Sanya underlines Beijing's desire to assert tighter control over this region.

"China's increasing dependence on imported petroleum and mineral resources has contributed to an intensified Chinese concern about defending its access to vital sea lanes, particularly to its south.

"It is this concern that in large part is driving China's development of power-projection naval forces such as aircraft carriers and long-range nuclear submarines."