Clandestine caves beneath the South China Sea off the Hainan island's coast could boost China's military strategic capabilities.

These underwater caves allow Chinese submarines unfettered access to routes in the disputed South China Sea, hidden from view of reconnaissance equipment, particularly those of the U.S. Navy, which has been a near-constant presence in the waters for close to 500 years, military analysts say, citing satellite images of the region.

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The fleet of Chinese submarines in the area signals President Xi Jinping's drive to secure sea lanes crucial in sustaining the country's growth and stability. It also hints at Beijing's goal to train its military to fight in the information age.

This has sparked uneasiness among neighbor states even as they scramble to augment their own underwater and reconnaissance capabilities.

Countries are concerned about the Chinese navy's growing power and are seeking credible deterrents for it, said Bill Hayton, author of "South China Sea: the struggle for power in Asia."

The fact that these nations are spending billions buying anti-ship missiles and submarines clearly shows what they're thinking, he said.

The Chinese Navy reportedly has 56 attack submarines and three nuclear-powered ones capable of launching ballistic missiles with a range of at least 4,600 miles (7,400 kilometers). This range hypothetically allows a missile to reach California if launched from the mid-Pacific or reach Hawaii if fired from the Western Pacific, said Dean Cheng of The Heritage Foundation.

The Pentagon believes China is looking to acquire five more of these submarines.

China claims nearly 90 percent of the South China Sea which is disputed by its Asian neighbors including Vietnam and the Philippines. In light of Beijing's military modernizations, nearby countries have sought to keep up.

Vietnam has acquired three Russian kilo-class submarines. Another three is expected to be received by 2016. Earlier this week, it signed a deal with India for four naval vessels believed to be used for reconnaissance missions in the South China Sea.

India is also looking to add 15 submarines to its fleet with a reported budget of about 800 billion rupees (US$ 13 billion). It received its fifth Boeing P-8I maritime surveillance aircraft last month.

TagsTerritorial disputes in the South China Sea, military spending