A top U.S. Navy official reportedly has seen increased Chinese activity around a reef in the disputed South China Sea region that Beijing seized from the Philippines almost four years ago.

Admiral John Richardson, the head of U.S. naval operations, told Reuters on Thursday that the U.S. saw Chinese activity around the Scarborough Shoal in the northern part of the Spratly Island chain – about 125 miles west of a Philippines military base.

"I think we see some surface ship activity and those sorts of things, survey type of activity, going on. That’s an area of concern ... a next possible area of reclamation," Richardson added.

Richardson expressed concerns that an international court ruling on China’s aggression in the region could prompt Beijing to declare an exclusion zone in the busy trade route. He said it was unclear whether the activity near the shoal was related to the pending decision.

China’s massive land reclamation and the creation of artificial island reefs threatened to destroy years of open access trading with Asian countries and could start new rules requiring permission to access those waters, Richardson warned. At least 30 percent of the world’s trade passes through that region.

He said the U.S. plans to continue carrying out freedom-of-navigation patrols within 12 nautical miles of the islands in the South China Sea and would welcome the participation of other countries in joint patrols.

However, he urges Washington to be careful about using “heavy” patrols of the region given China’s influence in the region.

"We have to be sophisticated in how we approach this so that we don’t force any of our partners into an uncomfortable position where they have to make tradeoffs that are not in their best interest.”

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