The United States warned of rising tensions in the South China Sea after China appeared to have placed a surface-to-air missile system on a disputed island.

Taiwan’s defense ministry said that China had positioned anti-aircraft missiles on Woody Island in the Paracel chain, which is occupied by China but also claimed by Taiwan and Vietnam.

State Department spokesman Mark Toner said today commercial satellite imagery appeared to indicate China has deployed a surface-to-air missile system. Another U.S. official gave a more direct confirmation of the deployment on Woody Island. The official, who was not authorized to discuss the information publicly and spoke on condition of anonymity, said it is unclear whether the deployment is intended for the long-term.

The deployment follows China’s building of new islands by piling sand atop reefs and then adding airstrips and military installations. The buildup is seen as part of Beijing’s efforts to claim virtually the entire disputed sea and its resources, which has prompted some of its wary neighbors to draw closer to the U.S.

In Beijing, Chinese Foreign Minister Wang Yi accused the media of hyping the issue and saying more attention should be paid to the “public goods and services” provided by China’s development of its maritime claims.

China’s actions in the South China Sea have becoming a source of tension not just with other Asian governments that claim territory there, but with Washington. Secretary of State John Kerry said the signs of increasing militarization contradicted a public assurance from Chinese President Xi Jinping when he visited the White House last September.

“When President Xi was here in Washington, he stood in the Rose Garden with President Obama and said China will not militarize the South China Sea. But there is every evidence every day that there has been an increase in militarization,” Kerry said before meeting with Poland’s foreign minister in Washington.

“It’s a serious concern,” he said, adding that he expected the U.S. would have a “very serious conversation” with China on the issue in the next few days.

U.S. network Fox News reported that China had moved two batteries of the HQ-9 surface-to-air missile system, along with radar targeting arrays on Woody island.

HIS Jane’s Intelligence Review agreed with that conclusion in its assessment of commercial satellite imagery of the island. The review’s deputy editor Neil Ashdown said that depending on the version of the HQ-9 deployed, the system has a range of between 78 miles and 143 miles, and would be the most advanced surface-to-air missile system currently deployed on land in the South China Sea. He described that as a significant military escalation.

Reports of the deployment came shortly after President Barack Obama wrapped up a summit in California on Tuesday with Southeast Asian leaders, who called for the peaceful resolution of the region’s maritime disputes through legal means.

Obama said the leaders had 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.”

That has been a frequent appeal from Washington in the past two years, but to little effect.

U.S. officials say China has reclaimed 3,200 acres of land, mostly in the Spratly Island group and has recently conducted test flights to an island there with a newly built 10,000-foot airstrip. The Paracels lie further north.

Although not one of the six governments with territorial claims in the South China Sea, the U.S. says it has a national interest in the region’s stability and freedom of navigation and overflight in and above what are some of the world’s busiest sea lanes.

Republican Sen. John McCain, chairman of the Senate Armed Services Committee, said Wednesday that China’s actions demonstrated Beijing’s desire to resort to coercion and President Xi’s “cavalier disregard for his public commitments to the United States.”

He said the U.S. should consider “raising the costs for Beijing.”

Called Yongxingdao by China, Woody Island has an artificial harbor, an airport, roads, army posts and other buildings. Recent satellite imagery appears to show it is adding a helicopter base likely dedicated to anti-submarine warfare missions.

China’s move is likely to rattle Vietnam the most because of its proximity to the Paracels and because of a history of maritime tensions with China that spiked in 2014 with a standoff after China moved a massive oil rig there.