Southeast Asian countries have altered a statement to be issued at Saturday's ASEAN summit to include references to militarization and island-building in the South China Sea, the latest draft shows, in a move likely to frustrate Beijing.

Chinese embassy representatives in Manila had sought to influence the content of the communique by lobbying Philippine officials, two ASEAN diplomatic sources told Reuters.

However, four ASEAN member states disagreed with omitting "land reclamation and militarization" - terms included in the statement issued last year in Laos, but not featured in an earlier draft of this year's statement seen on Wednesday.

China is not a member of the Association of South East Asian Nations, and is not attending the summit. China embassy officials in Manila could not be reached and China's foreign ministry did not immediately respond to request for comment.

ASEAN references to the South China Sea issue typically do not name China. Beijing is extremely sensitive to anything it perceives as a veiled reference to its expansion of its seven manmade islands in the Spratly archipelago, including with hangers, runways, radars and missiles.

The final version of the statement has yet to be agreed, but changes so far indicate ASEAN is resisting moves by China to keep its contentious activities in the strategic waterway off ASEAN's official agenda.

China's lobbying, and its burgeoning friendship with Philippine President Rodrigo Duterte, may not have been enough to influence Manila's position either.

"The Philippines is under too much pressure," one of the sources said.

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This year's summit comes at a time of uncertainty about U.S. interests in the region and whether it will maintain its maritime presence to counter China's assertiveness.

Chinese officials pressed for words that might allude to last year's international arbitration ruling to be kept out of the statement, the diplomats said, particularly the term "full respect for legal and diplomatic processes".

The latest draft still includes that, although it was moved out of the South China Sea section to another.

"They do not want any phrase linked to the arbitration case," one source said.

The Hague ruling, in a case brought by the Philippines in 2013, angered China because it invalidated China's claim of sovereignty over almost the entire South China Sea. China refuses to recognise the decision.

As part of his engagement with China, Duterte has decided not to press it to abide by the arbitration award anytime soon. On Thursday he said it was pointless for ASEAN to pressure China.

In his address to open the leaders' summit, Duterte made no mention of the South China Sea, but touched on many issues central to his 10-month administration.

Duterte mentioned extremism, piracy, interference in a country's affairs, and his signature fight against drugs, for which he has been widely condemned over the deaths of thousands of Filipinos.

"The illegal drug trade apparatus is massive. But it is not impregnable," he said. "With political will and cooperation, it can be dismantled, it can be destroyed before it destroys our societies."

Duterte then hosted two meetings with ASEAN leaders, which were not open to media.

ASEAN and China are hoping to this year agree on a framework to create a code of conduct over the South China Sea, 15 years after committing to draft it. Some ASEAN diplomats doubt China is sincere about agreeing to a set of rules.

In unusually direct comments for an ASEAN Secretary General, Le Luong Minh on Thursday told Reuters the code needed to be legally binding to put a stop to "unilateral actions", because a previous commitment to play fair had been ignored.