BEIJING — For months, China's visible construction of artificial islands and military facilities in the South China Sea has alarmed U.S. officials and many of China's neighbors.
What is happening under the water is also worrisome, say several defense and security analysts.
China has a growing fleet of nuclear submarines armed with ballistic missiles. The expansion of its claim on the South China Sea may be intended to create a deep-water sanctuary — known in military parlance as a "bastion" — where its submarine fleet could avoid detection.
"The South China Sea would be a good place to hide Chinese submarines," said Carl Thayer, a U.S.-born security specialist who has taught at the University of New South Wales and other Australian institutions. The sea floor is thousands of meters deep in places, with underwater canyons where a submarine could easily avoid detection.
Conflicts in the South China Sea are expected to be a major focus of annual U.S.-Sino talks that start Tuesday in Washington, including meetings between U.S. Secretary of State John Kerry and Chinese Vice Premier Wang Yang.
China last week announced that it was winding down its expansion of artificial islands in the South China Sea, but the statement wasn't warmly received by U.S. officials.
Daniel Russel, assistant U.S. secretary of state for East Asian and Pacific affairs, noted that China continues to build facilities on the islands, including military installations, a move that he said was "troubling."
"The prospect of militarizing those outposts runs counter to the goal of reducing tensions." Russel said Thursday during a briefing in Washington. "That's why we consistently urge China to cease reclamation, to not construct further facilities, and certainly not to further militarize outposts in the South China Sea."
The South China Sea — bounded by Vietnam, China, Taiwan, Japan, the Philippines and Malaysia — is one of the world's most important shipping lanes. China asserts it holds maritime rights to 80 percent of the sea, a claim that other countries have vigorously contested.
According to Thayer, Beijing sees the South China Sea as a strategic asset because it guards China's southern flank, including a submarine base in Sanya, on China's Hainan island. The People's Liberation Army Navy has built underwater tunnels there to quietly dock some of its submarines, including those that carry ballistic missiles.
As of 2014, China had 56 attack submarines, including five that were nuclear powered. It also has at least three nuclear-powered submarines capable of launching ballistic missiles, and is planning to add five more, according to a Pentagon report released last year.
In an April media briefing in Washington, a top U.S. Navy official said the Pentagon is watching China's ballistic submarines "very carefully."
"Any time a nation has developed nuclear weapons and delivery platforms that can range the homeland, it's a concern of mine," said Adm. William Gortney, the commander of the U.S. Northern Command. Gortney quickly added that China has a policy of "no first use" of nuclear weapons, "which gives me a little bit of a good news picture there."
In recent decades, China has worked to build up a nuclear deterrence capability in the shadow of that developed by the United States and Russia. Its submarine program is a major part of that push. Since submarines can often avoid detection, they are less vulnerable to a first-strike attack than land-based intercontinental ballistic missiles or nuclear bombers.
Currently, China's JL2 submarine ballistic missiles lack the capacity of reaching the continental United States from the South China Sea. But China hopes to improve the range of those missiles in coming years, which is why analysts think China sees the sea as a future "bastion" for its nuclear submarines.
Bernard D. Cole, a professor at the National War College and a retired U.S. Navy captain, says the Soviets developed the submarine bastion strategy during the Cold War. A spy ring alerted the Soviets to the fact that the United States was easily tracking their submarines in the open ocean. So the Soviets created heavily mined and fortified zones for their subs to operate as close to the United States as possible. One was in the White Sea of northwest Russia and the other was in the Sea of Okhotsk, north of Japan, said Cole.
Chinese submarines are known for being relatively noisy — and thus easy to detect — making it difficult for them to slip into the western Pacific without being detected. But once China improves the range of its missiles, it won't need to move its submarines out of the South China Sea to pose a retaliatory threat to the United States.
"My own conclusion, right now, is that China will adopt a bastion strategy in the South China Sea," Cole said in an email, noting he was expressing his personal views, not those of the National War College. China's bastion strategy, he said, will bank on fairly rapid development of ballistic missiles with the range to reach the United States.
U.S. officials are concerned that China might unilaterally declare an "air defense identification zone" in the South China Sea that would restrict military overflights, including U.S. planes attempting to track China's submarines. Last month, when a U.S. surveillance plane carrying a CNN crew flew over some of the islands, the Chinese navy issued urgent warnings to back off, a possible sign of things to come.
The two-day U.S.-China Strategic and Economic Dialogue, an annual exchange between the two countries, starts Tuesday in Washington. China and U.S. officials will discuss trade and economic issues, and the U.S. side will likely raise concerns over recent cyber theft of federal employee data, thought to originate from China.
In the run-up to the meeting, Chinese state media has been playing down tensions between the two countries.
"Following months of diplomatic clashes over the South China Sea, Sino-U.S. relations seem to be headed for calmer waters," China Daily reported Friday.
Thayer and other analysts say China has multiple reasons for building its artificial islands in the South China Sea. One purpose is to intimidate neighbors, particularly Vietnam and the Philippines.
"China hopes to put pressure on the Philippines so it will not provide the U.S. with a rotational (military) presence," said Thayer. In May of 2016, Filipinos will vote in a presidential election that could determine the future of U.S. military access to the Philippines.