A Chinese Coast Guard ship, in the background, is seen near a Vietnam Marine Guard ship in the South China Sea, about 130 miles off the coast of Vietnam, in May 2014. (Nguyen Minh/Reuters)

The Obama administration has been forcefully trying for nearly six years to prod China to abide by the usual international rules and to curb Beijing’s controversial claims of Chinese sovereignty over a maritime corridor in Southeast Asia.

But new evidence is emerging that China has been playing a familiar game of ignoring U.S. warnings or concerns, this time by installing radar, missiles and jet fighters on a series of reefs and atolls over which it has asserted control in the South China Sea. The long-simmering dispute in this far-flung spot on the globe represents yet another test for President Obama’s foreign policy in this final year of his presidency.

The disclosure of China’s maneuvers — coming a week after Obama declared that the United States “will continue to fly, sail and operate wherever international law allows” — prompted the top U.S. military commander in the Pacific to warn Congress that China’s ultimate goal is to achieve “hegemony” in Asia. Foreign policy analysts in Washington cautioned that Beijing is on the verge of tilting the security and economic dynamics along the South China Sea in its favor.

Obama’s strategy abroad has been to focus on building partnerships in hopes of slowly turning the global order to U.S. advantage, but China favors a more confrontational approach and is pursuing tactical victories that are adding up to a competitive edge.

Some U.S. military planners fear that edge is coming into clearer view. Over the past year, China’s military operations have expanded in size, complexity, duration and geographic location, Marine Gen. Joseph F. Dunford, chairman of the Joint Chiefs of Staff, told the House Appropriations Committee on Thursday. He testified that China has specifically developed capabilities that counter U.S. strengths, including ballistic and cruise missiles that would help protect against U.S. aircraft.

“China’s rapid military modernization is quickly closing the gap with U.S. military capabilities and is eroding the joint force’s competitive military advantages,” Dunford said. “China’s military forces can constrain U.S. military operations in the Western Pacific and hold key U.S. infrastructure and facilities at risk. Its strategic capabilities are improving and present an increasing risk to the U.S. homeland and our allies.”

White House officials played down the severity of Beijing’s latest moves and emphasized that the administration will continue to pursue a diplomatic resolution to the dispute, which has embroiled China’s neighbors, including Vietnam, the Philippines and Taiwan.

Obama met with 10 Southeast Asian leaders during a summit in California two weeks ago, and he said they discussed “the need for tangible steps in the South China Sea to lower tensions, including a halt to further reclamation, new construction and militarization of disputed areas.”

But a joint statement on the freedom of navigation signed by all the participants did not specifically mention China or the South China Sea, a sign that the nations fear being caught in a geopolitical struggle between two major powers.

During a visit to Washington last week, Chinese Foreign Minister Wang Yi made the rounds of the White House, Capitol Hill and the State Department. Obama dropped in unannounced on Wang’s meeting with national security adviser Susan Rice.

Wang, during an appearance at a think tank, dismissed the notion that China was pursuing an unlawful expansion of power and suggested that it was Washington, not Beijing, that was overstepping its bounds. The South China Sea represents a legitimate national interest for China, he said, and claimants in the region should work out any conflicts among themselves without input from “outside countries.”

“China will not become another United States,” Wang said at the Center for Strategic and International Studies (CSIS). “In the blood and veins of China, there are no veins of expansionism. There is no mentality or urge to be saviors of the world. Starting 2,000 years ago, we built the Great Wall for self-defense. That is the special characteristic . . . of Chinese culture.”

One-third of the world’s shipped goods pass through the South China Sea each year, and the area is thought to contain abundant reservoirs of oil and natural gas. U.S. military planners consider free passage a strategic imperative to defend the United States’ treaty allies in Asia.

The Obama administration entered the fray in 2010 after China sparked a row with Vietnam by forcing Exxon Mobil to stop oil exploration efforts in offshore areas claimed by Hanoi. During a regional conference in Hanoi that summer, then-Secretary of State Hillary Clinton announced that the resolution of the territorial disputes in the South China Sea was “a leading diplomatic priority” for the Obama administration.

The statement was widely considered a shift of the administration’s China policy toward a harder line and a signal that the United States would seek to deepen ties in Southeast Asia as a hedge to China’s growing clout.

In the ensuing years, Obama deployed of a small contingent of Marines to Darwin, Australia; announced new security partnerships with the Philippines; declared that the United States would defend Japan in a separate clash with China over disputed islands in the East China Sea; and met annually with the leaders of the Association of Southeast Asian Nations.

Instead of backing down, however, China embarked on a massive land reclamation project to build up tiny reefs and atolls in the Spratly Islands by dredging sand from the ocean floor. Beijing said the costly project was benign, but U.S. officials charged that the construction of jet-plane landing strips on the virtually deserted islands meant that China was readying the parcels for military use.

China’s declaration of sovereignty over the Spratlys outraged the Philippine government, which took the case to an international tribunal at the Hague. Obama administration officials view that ruling, expected later this year, as a potential turning point, even though Beijing does not recognize the Hague’s authority on the matter.

In the meantime, the U.S. Navy has conducted two “freedom of navigation” maneuvers, sending warships within 12 nautical miles of several disputed islands.

“The administration is waiting for the Hague’s legal decision in the Philippines challenge before taking another strong step,” said retired Navy Capt. Jerry Hendrix, a defense analyst at the Center for a New American Security. “There’s a strong emphasis on the legal mechanisms to constrain China, and a desire not to muddy the water before the Hague’s decision.”

But Hendrix cautioned that “every day that goes by without a really significant challenge on China’s claims on the island is a lost opportunity. They are building a precedent that this is the new status quo, and then reversing it is difficult.”

In the White House, China is considered a major rival to the United States — but also a crucial global partner. Obama has enlisted Beijing’s support on issues such as the Iran nuclear deal and a major pact to reduce carbon emissions. Last week, China joined the United States in support of a U.N. Security Council resolution to enact new economic sanctions against North Korea because of its nuclear test in January.

Inside the West Wing, aides have been divided over how hard to push Beijing on areas of disagreement, including cybersecurity, human rights and the South China Sea, for fear of damaging the cooperation on other matters.

Last fall, during a state visit to the White House, Chinese President Xi Jinping stood next to Obama in the Rose Garden and vowed that China would not militarize the Spratly Islands. But last week, the CSIS think tank published satellite images that analysts said showed that China was installing a powerful radar system in the Spratlys.

Separately, U.S. officials said China had deployed surface-to-air missiles and jet fighters on Woody Island, which is closer to China and has been under its control for decades.

Foreign policy analysts in Washington said the Chinese have mastered the ability to take incremental steps that fall just short of provoking a major international incident — a process known as “salami-slicing,” or taking territory piece by piece until everything is gone.

Administration officials point out that China’s intimidation has pushed several neighbors into greater partnerships with the United States. Vietnam, Malaysia and Brunei are members of the 12-nation Trans-Pacific Partnership trade pact led by the Obama administration. In 2012, Obama restored formal U.S. diplomatic relations with Burma, also known as Myanmar, after 50 years of estrangement, and he will become the first U.S. president to visit Laos in the fall.

Whether any of it matters is still unknown.

“People in the Obama administration will tell you that China is creating its own enemies and that they are unpopular,” said Michael Green, who served as senior Asia director in the George W. Bush administration. “That may be true, but that’s not doing anything to slow them down. They do not care as much about reputation as we do.”