Beijing's claim to almost the entire resource-rich sea is shown on Chinese maps with a nine-dash line that stretches deep into the maritime heart of Southeast Asia. Vietnam, the Philippines, Taiwan, Malaysia and Brunei also claim parts of the waterway.

Last year, the Indonesian armed forces chief accused China of including parts of the Indonesian-ruled Natuna islands within the nine-dash line.

Indonesian President Joko Widodo's administration departed from its usual low-profile role in the dispute on Wednesday when security chief Luhut Panjaitan said Jakarta could take China to an international court if dialogue over the islands failed.

"The position of Indonesia is clear at this stage that we do not recognise the nine-dash line because it is not in line with international law," Indonesian Foreign Ministry spokesman Armanatha Nasir told reporters.

"We asked for clarification on what they mean by the nine-dash line. That has not been clarified."

Nasir could not say when the request through diplomatic channels was made to China.

Officially, China and Indonesia do not contest the sovereignty of the Natuna islands: both agree they are part of Indonesia's Riau province. Nor does Indonesia challenge Beijing's expansive claims in the South China Sea.

The Philippines has taken China to the Permanent Court of Arbitration in the Hague, a case Beijing refuses to recognise. For years, China has insisted that disputes with rival claimants be handled bilaterally.

When asked if Indonesia could also take China to court, as Panjaitan had said, Nasir responded: "We cannot preempt things before we know how they evolve. But what is clear is that we are not a claimant state and we don't recognise the issue of the nine-dash line, which we have made clear to China."

Regional leaders are expected to discuss the issue at the meeting of the Association of Southeast Asian Nations (ASEAN) later this month.

REUTERS