The dispute over the South China Sea just got a bit more vexed.

Jakarta's decision to summon the Chinese envoy to protest against a Chinese coast guard vessel's harassment of Indonesian boats attempting to corral a fishing vessel poaching in waters off the Natunas has brought Indonesia closer towards being the fifth South-east Asian party to the dispute, joining Vietnam, the Philippines, Brunei and Malaysia.

The difference is that China is no longer dealing with a minnow; Indonesia is Asean's biggest nation and economy. That Beijing has chosen to rub up against it suggests it was testing Jakarta's reflexes - as it did with Vietnam over an oil rig - and that its strategic objectives outweigh considerations of medium-term unpopularity with the region.

Jakarta has been on watch mode following China's nine-dash-line claim, which loops down, like a cow's tongue, towards its Natuna Islands. For a long time, Beijing had maintained an ambiguous stance on the Natunas but last November, the Foreign Ministry said it had no claim on the islands themselves.

However, it remained silent on the 200km exclusive economic zone (EEZ) that surrounds these waters. Privately, the People's Liberation Army (PLA) in Beijing has maintained that Indonesia is sitting on 50,000 sq km of its maritime territory around the Natunas.

The weekend incident and yesterday's Foreign Ministry comment that the poachers did not enter Indonesia's "territorial waters", which extend at most to 12 nautical miles, suggest that Beijing is reluctant to concede Indonesia its EEZ. Indeed, it says the offending boat was in "traditional Chinese fishing grounds".

Jakarta had taken comfort from the soothing words of Chinese diplomats. But the China Coast Guard's actions suggest the PLA boys are determined to assert themselves on the issue, never mind if it falls to the Foreign Ministry to clean up after them.