"With both China and the US unlikely to change course, more close calls and increasing tensions are likely," the Soufan Group noted, calling the trend "increasingly worrisome."
On May 10, the US sent its guided missile destroyer USS William P. Lawrence into the 12-mile radius of the Fiery Cross Reef, known in China as the Yongshu Island. This was the Pentagon's third naval operation in the region claimed by China, Taiwan, the Philippines, Vietnam and several other countries.
The US carried out the operation to "challenge excessive maritime claims" by the claimants, who allegedly want to "restrict the navigation rights" in the South China Sea. Beijing denied this claim.
"For a long time, the freedom of navigation and overflight in the region has been upheld thanks to the cooperative and concerted efforts by China and relevant coastal countries in the South China Sea," Chinese Foreign Ministry Spokesperson Lu Kang told reporters on Tuesday.
"In fact, the freedom of navigation and overflight and the passage of vessels of all countries have never met any obstacle," he added.
In late October, the US carried out its first FONOP mission when the USS Lassen sailed close to Subi Reef, an islet in the Spratly archipelago. Brunei, China, Malaysia, the Philippines, Taiwan and Vietnam have been locked in a territorial dispute over the island chain.
In late January, the USS Curtis Wilbur sailed within 12 nautical miles of the Triton Island in the Paracels, an archipelago claimed by China, Taiwan and Vietnam. The US did not notify any claimant prior to the operations.