LONDON — China secretly has built a major underground nuclear submarine base that could threaten Asian countries and challenge American power in the region.

Satellite imagery, passed to the Daily Telegraph, shows that a substantial harbor has been built that could house a score of nuclear ballistic missile submarines and a host of aircraft carriers. In what will be a significant challenge to American naval dominance and to countries ringing the South China Sea, one photograph shows China’s latest 094 nuclear submarine at the base only a few hundred miles from its neighbors.

Other images show numerous warships moored at long jetties and a network of tunnels at the Sanya base on the southern tip of Hainan island.

Of even greater concern to the Pentagon are immense tunnel entrances, estimated to be 60 feet high, carved into hillsides around the base. Sources fear they could lead to caverns capable of concealing from spy satellites up to 20 nuclear submarines.

Satellite evidence shows 11 tunnel openings at the base. Analysts believe that the ones photographed could be opened to the sea following further construction work.

Analysts have also detected a possible demagnetization facility at the dockside that would make a submarine much harder to detect by sonar.

The U.S. Defense Department has estimated that China will have five 094 nuclear submarines operational by 2010, with each capable of carrying 12 JL-2 nuclear missiles.

The images were obtained by Janes Intelligence Review after the periodical was given access to imagery from the commercial satellite company DigitalGlobe.

Analysts for the respected military magazine suggested that the base could be used for “expeditionary as well as defensive operations” and would allow the submarines to “break out to launch locations closer to the U.S.”

It would now be “difficult to ignore” that China was building a major naval base where it could house its nuclear forces and increase its “strategic capability considerably further afield.”

The development, close to sea lanes vital to Asian economies, “can only cause concern far beyond these straits.”

Military analysts believe that China’s substantial build-up of its forces is gaining pace but has remained hidden from the world during the preparations for this summer’s Olympic Games in Beijing.

China has diverted much of its resources from the huge People’s Liberation Army to the navy, air force, and missile development. An old Russian aircraft carrier, bought by Beijing for “leisure activities” has been picked over by naval architects who hope to “reverse engineer” the ship.

Within the next five to 10 years, China is expected to build up to six carriers, which will coincide with the Royal Navy’s construction of two major carriers.

The location of the base off Hainan will also give the submarines access to very deep water — exceeding 15,000 feet — within a few miles, making them even harder to detect.

Britain’s Trident submarines have to remain on the surface when they leave Faslane in northwest Scotland and cannot dive to depth until they are outside the Irish Sea.

While it has been known that China might be developing an underground base at Sanya, the pictures provide the first proof of its existence and the rapid progress made. Two 1,000-yard piers and three smaller ones could accommodate two carrier strike groups or amphibious assault ships.

The editor of Jane’s Intelligence Review, Christian Le Miere, said the complex underlined Beijing’s plan “to assert tighter control over this region.” He added: “This is a challenge to any hegemonic power, particularly the U.S. which remains dominant in the region.”

So far China has offered no public explanation for its building at Sanya. The state bills the tropical island of Hainan as a new Hawaii. Its sparkling beaches are lined by hotels patronized by western expatriates, Russian package tours, and China’s new middle class.

Sanya is best known for hosting Miss World in recent years. But right next door, China’s forward-looking naval strategists are putting a different vision of international relations into effect.