The summit failed to issue a closing communique after the United States and Beijing furiously disagreed over the wording of trade issues, leaving a cloud of uncertainty and bitterness, and strained relations in our own backyard.
Speaking to media in Port Moresby following the leaders' impasse, O'Neill said there were "two big giants in the room".
A statement will later be issued by the meeting's chairman, Papua New Guinea's Prime Minister Peter O'Neill.
While tariffs between the US and China have led to conflict in the past year, the summit also showed the U.S.is trying to compete with China as both superpowers pledge more aid to developing countries.
Washington had pressed for leaders to issue what amounted to a denunciation of the World Trade Organization and a call for its wholesale reform - a step too far for Beijing, which would likely suffer in the event of any change to trade rules.
Chinese President Xi Jinping called on Asia-Pacific Economic Cooperation (APEC) economies to bring their cooperation to a higher level based on a keen appreciation of the trend of the world and its economy.
The World Trade Organization (WTO) is exploring a new round of reform, Xi noted, saying such reform should be created to uphold the core values and underlying principles of the multilateral trading system.
Canada's Prime Minister Justin Trudeau said there were differences between several nations including China and the US.
"There are always disagreements behind the scenes at these sorts of summits, but normally by the end, publicly, a document is published that all the leaders can put their names to", he said.More news: Czechs take 2-0 lead against United States in Fed Cup final
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"Throughout the ASEAN Summit and Related Summits and the APEC Economic Leaders' Meeting, President Duterte continued to advance Philippine national interests and prioritized upholding and promoting the Philippines key positions".
Mr Lee's comments came as trade tensions between China and the US dominated the annual summit in Papua New Guinea over the weekend, with Mr Xi and US Vice-President Mike Pence trading barbs in back-to-back speeches on Saturday.
Prior to Pence's comments, China's Xi Jinping lashed out at U.S. protectionism, labeling the USA approach as "short-sighted" and was "doomed to failure".
On Sunday, the US, Japan, New Zealand, and Australia said they would invest $1.7 billion in the Papuan economy to help the country develop a modern electric power grid - concurrent to Chinese investment in the region. "Presume good faith and good intentions on the other party, and work out practical solutions to those problems", he said, calling it a "chicken and egg problem" that requires a level of trust for both sides to start to resolve.
China has so far retaliated with tariffs on $110 billion of US products and is likely to respond with more if the United States goes ahead with the increase at the start of January.
During the APEC summit, Australia, Japan and the USA resisted Chinese efforts to use spending through Beijing's Belt and Road initiative to gain influence in the Pacific.
The "opaque" loans come with strings attached and build up "staggering debt", Pence charged, mocking the initiative as a "constricting belt" and a "one-way road".
Pence accused China of luring developing nations into a debt trap through the loans it offered for infrastructure.
"The assistance provided by China has been warmly welcomed by our partners in this region and beyond", Wang Xiaolong, a foreign ministry official, told a news conference.
"Attempts to form exclusive blocs or impose one's will on others should be rejected", Xi said.