US President Donald Trump's national security adviser called Beijing's claims in the South China Sea "ridiculous" and announced upcoming meetings with his counterparts in Japan, India and Australia to strengthen defence partnerships in the region.
Robert O'Brien said China's claim to nearly the entire ocean that Vietnam, Indonesia, the Philippines and other Southeast Asian countries claim part of has been "rejected by all major countries, all seafaring countries", in an online discussion with Paula Dobriansky, vice chair of the Atlantic Council's Scowcroft Centre for Strategy and Security.
"It's been rejected by [a tribunal for] the Law of the Sea, and now China's engaged in military exercises in these waters that are, that they consider domestic, and which are by no stretch of the imagination domestic," he said, referring to a 2016 ruling by a tribunal at The Hague, which determined China had no "historic rights" over the sea.
"The United States is not going to back down from its long-held principles that the world's oceanways and international waters should be free for navigation, and the same with space and with air rights in international airspace."
Beijing rejected the tribunal's ruling and has described it as having "no binding force".
O'Brien was responding to questions from Dobriansky, a former under secretary of state for global affairs, about China's accusation that a US U-2 spy plane entered a no-fly zone without permission during a Chinese live-fire naval drill off the country's northeast coast and news that China subsequently launched two missiles, including an "aircraft-carrier killer", into the area.
Disputes between Beijing and Washington over the South China Sea have been escalating since Beijing began land-reclamation operations in 2016 in some of the features it controls in the ocean's Spratly archipelago, and built a new city on one of the islands - Sansha on Woody Island - in turn leading to an increase in Chinese tourism.
The Trump administration added friction by issuing a direct challenge to China's claims by calling them "completely unlawful".
O'Brien said high-level meetings of "the quad", comprising the US, Japan, Australia and India were being planned for September and October.
"I think the quad, which is really coming into its own ... is one of the most exciting diplomatic initiatives and ... one of the areas most likely to succeed and pay huge dividends in the future," he said. "So I'll be meeting with my quad partners, likely in Hawaii in October, with the [national security advisers] of those countries."