A U.S. Marine MV-22 Osprey maneuvers over military tanks during joint military exercises at Crow Valley in Capas, Philippines, on Thursday. (Francis R. Malasig/European Pressphoto Agency)

The United States announced Thursday it has started joint patrols with the Philippines in the disputed waters of the South China Sea and is ramping up its military presence in the area, a move that China immediately denounced as “Cold War thinking.”

In Manila, Defense Secretary Ashton B. Carter said the first two patrols took place in March and April and that missions would continue on a regular basis.

[Possible tensions below the waves as well]

The deployment comes amid growing concerns about Chinese reach in the strategically and economically important waters of the South China Sea, which borders the Philippines, Vietnam and several other nations.

Beijing has built artificial islands and runways on seven disputed reefs, raising fears that China could seek new military outposts in the region. The latest U.S. moves are certain to boost tensions.

China has laid claim to a number of islands in the South China Sea, building airbases on tiny spits of land while installing powerful radar and missile launchers. Here's why. (Jason Aldag,Julie Vitkovskaya/The Washington Post / Satellite photos courtesy of CSIS)

“Countries across the Asia-Pacific are voicing concern with China’s land reclamation, which stands out in size and scope, as well as its militarization in the South China Sea,” Carter told reporters, according to Reuters news agency. Carter said he will be visiting the aircraft carrier USS John C. Stennis in the South China Sea on Friday.

About 300 U.S. troops, including Air Force crews with combat aircraft and helicopters, will stay in the Philippines through the end of the month.

They are among a contingent already taking part in 11 days of combat exercises, the Associated Press reported.

Troops and military equipment will also be sent on regular rotations to the Philippines.

[Pentagon chief puts off China visit]

China, in turn, accuses the United States of militarizing the region as part of President Obama’s strategic rebalance to Asia. It has also pointed to a 2011 deal to station Marines in Darwin, Australia, and a recent announcement allowing U.S. troops access to five bases in the Philippines.

“Reinforcing a military alliance is a sign of Cold War thinking, and it runs against the trend of this era — peace, development, cooperation and win-win,” the Ministry of National Defense told China Daily in response to the latest news.

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Tensions between China and the Philippines have been steadily building ahead of a ruling expected soon from an international tribunal in The Hague on the South China Sea dispute.

The government in Manila initiated proceedings after China seized control of Scarborough Shoal in the Spratly Islands in 2012. China says that it will not recognize the tribunal’s decision and that all disputes must be settled bilaterally.

[The covert war led by Chinese fishermen]

The Philippine government worries that China could begin a new program of land reclamation and building on the shoal, which lies 145 miles west of the Philippines and 620 miles from the Chinese coast. The Philippines’ ambassador to Washington, Jose L. Cuisia Jr., recently appealed for U.S. help to block any Chinese expansion.

While the United States insists its growing support for allies in the Asia-Pacific region is not aimed at any one nation, officials have openly expressed concerns over Beijing’s actions in the area. Adm. Harry Harris Jr. told Congress in February that China aimed to achieve regional “hegemony.”

Last week, the Pentagon announced $40 million in military assistance to the Philippines to beef up intelligence sharing, surveillance and naval patrols.

China has built airstrips and appears to be building a sophisticated military radar system in the Spratlys. It also stationed surface-to-air missiles and fighter jets in the Paracel Islands — a separate chain over which China has enjoyed full control since a 1974 naval battle with Vietnam but whose sovereignty Hanoi still bitterly contests.

[Satellite images reveal possible Chinese radar sites]

China says those deployments are only for defensive purposes on its sovereign territory.

It also argues that the United States did not complain when the Philippines and Vietnam reclaimed land or built airstrips in the South China Sea in the past.

Beijing reacted furiously this week to reports that the Philippines was upgrading its own airstrip in the Spratlys, accusing it of hypocrisy and trying to extend its “illegal” occupation.

In a testy exchange with reporters in Washington on Tuesday, State Department spokesman Mark Toner appeared reluctant to criticize Manila for its actions, pleading that he did not have details.

China also asserts that U.S. “interference” is destabilizing the region and warns that U.S. cooperation with the Philippines should not target a “third party.”

“I also want to mention that the U.S. military has kept talking about the so-called militarization in the South China Sea,” Foreign Ministry spokeswoman Hua Chunying said in a typical comment last month. “Maybe they can explain whether their increased military deployment in the South China Sea and nearby areas is an action of militarization or not?”

Chen Xiangmiao, a researcher at the National Institute for South China Sea Studies on China’s southern island of Hainan, said the latest U.S. move was expected but could inflame tensions if the patrols and the increased military presence are viewed as pressuring China.

“The United States should not just do things on impulse, which will cause strong resistance from the public in China and distrust among decision-makers here,” he said. “The distrust between the two may get deeper, and the conflicts will rise in a spiral.”

Gu Jinglu contributed to this report.

Today's coverage from Post correspondents around the world