Southeast Asian leaders head into a historic summit in Myanmar this weekend dogged by a flare-up of high-seas tensions with China that will test the region's ability to stand together against a mighty economic partner.
Vietnam and the Philippines, both members of the 10-nation Association of Southeast Asian Nations (ASEAN), squared up to Beijing this week in the South China Sea, whose waters are scored by overlapping territorial claims.
"China's actions on the eve of the ASEAN meeting in Myanmar will put South China Sea issues on the top of the agenda," said Carl Thayer, an expert on the region at the University of New South Wales.
He said Beijing had been "aggressively assertive" by relocating a deep-water drilling rig in waters claimed by Vietnam and surrounding it with ships, adding it could be a riposte to US President Barack Obama's recent Asia tour.
Hanoi said yesterday that the Chinese ships used water cannon to attack Vietnamese patrol vessels and repeatedly rammed them, injuring six people.
During his tour, Obama asserted support for US allies Japan and the Philippines, both locked in their own territorial disputes with Beijing.
Philippine police said yesterday they had seized a Chinese fishing vessel and detained its 11 crew members elsewhere in the South China Sea.
China said it was in the right in both the Philippine and Vietnam cases.
The sea is crisscrossed by fishing and shipping lanes and is thought to contain huge oil and gas reserves. Parts are claimed by ASEAN members Vietnam, Brunei, Malaysia and the Philippines, as well as by Taiwan.
China, which asserts sovereign rights to almost all of the disputed waters, wants to negotiate with rivals on a bilateral basis. Other claimants reject that and want a multinational approach.
"ASEAN is not likely to condemn China by name and will stick to its past formula of upholding international law, rejecting force or the threat of force, and call for an early conclusion of a binding code of conduct," Thayer said.