MANILA - Regional delegates expecting Philippine leadership in discussions on the South China Sea dispute were frustrated when President Rodrigo Duterte insisted China's arbitration defeat would not be on the agenda, a senior diplomat said Friday.

Some of them were preparing to bring up the Hague-based ruling as reference for discussions on a code of conduct in the disputed waters, said the envoy who is familiar with the talks.

But they were "precluded" by Duterte's statement Thursday that the ruling was "not an issue" in the meetings of the Association of Southeast Asian Nations (ASEAN), the official said.

"Your president has defined the outcome of ASEAN 50 already," the diplomat quoted one delegate as saying during a consulation meeting.

"Some are frustrated over the turn of events."

Duterte said ASEAN could tackle the code of conduct, but not the ruling which was "between China and the Philippines."

Chairing this year's ASEAN meetings is seen as an opportunity for the Philippines to rally the regional bloc behind a common stand on the maritime dispute.

But under Duterte, the Philippines has sought warmer ties with China in exchange for billions of dollars in aid and investment.

"It now appears that we're being lumped together with Cambodia and Laos in protecting Chinese interests (in ASEAN) at all costs," the diplomat said.

China has erected artificial islands in the disputed waterway, including a weapons system, despite a 2002 agreement with ASEAN to exercise "self-restraint" and avoid activities that could escalate tensions in the region.

Both sides have agreed to turn their existing Declaration on the Conduct of Parties in the South China Sea into a binding set of rules to manage tensions in the area.

A draft statement to be released Saturday made no reference to the arbitral ruling, which invalidated the basis of China's sweeping maritime claims.

The final document will likely just reiterate a 2016 statement, which "took note of the concerns" over land reclamations in the disputed waters, but made no mention of China.