Construction and dredging underway at Mischief Reef, a large reef in the Spratly Islands in the South China Sea on March 16, 2015. DigitalGlobe | Getty Images

The United States has seen Chinese activity around a reef that China seized from the Philippines nearly four years ago that could be a precursor to more land reclamation in the disputed South China Sea, the U.S. Navy chief said on Thursday. The head of U.S. naval operations, Admiral John Richardson, expressed concern that an international court ruling expected in coming weeks on a case brought by the Philippines against China over its South China Sea claims could be a trigger for Beijing to declare an exclusion zone in the busy trade route. Richardson told Reuters the United States was weighing responses to such a move. He said the U.S. military had seen Chinese activity around Scarborough Shoal in the northern part of the Spratly archipelago, about 125 miles (200 km) west of the Philippine base of Subic Bay. "I think we see some surface ship activity and those sorts of things, surve type of activity, going on. That's an area of concern ... a next possible area of reclamation," he said.

Richardson said it was unclear if the activity near the reef, which China seized in 2012, was related to the pending arbitration decision. He said China's pursuit of South China Sea territory, which has included massive land reclamation to create artificial islands elsewhere in the Spratlys, threatened to reverse decades of open access and introduce new "rules" that required countries to obtain permission before transiting those waters. He said that was a worry given that 30 percent of the world's trade passes through the region. Asked whether China could respond to the ruling by the court of arbitration in The Hague by declaring an air defense identification zone, or ADIZ, as it did farther north in the East China Sea in 2013, Richardson said: "It's definitely a concern." "We will just have to see what happens," he said. "We think about contingencies and  responses."

Richardson said the United States planned to continue carrying out freedom-of-navigation exercises within 12 nautical miles of disputed South China Sea geographical features to underscore its concerns about keeping sea lanes in the region open. Joint patrols? The United States responded to the East China Sea ADIZ by flying B-52 bombers through the zone in a show of force in November 2013. Richardson said he was struck by how China's increasing militarization of the South China Sea had increased the willingness of other countries in the region to work together, not just bilaterally, but also multilaterally. India and Japan joined the U.S. Navy in the Malabar naval exercise since 2014, and were slated to take part again this year in an even more complex exercise that will take place in an area close to the East and South China Seas. South Korea, Japan and the United States were also working together more closely than ever before, he said.