The sea, one of the world's major trade passageways, is considered a flashpoint between China and rival claimants with the USA also desperate to keep the waterway open to its vessels.
"Substantial progress has been made with 15 of the 16 RCEP countries committing to finalise minor outstanding matters within months and proceeding to signing this deal next year", he said.
Union Home Minister Amit Shah on Monday said India's decision not to sign the Regional Comprehensive Economic Partnership (RCEP) reflects Prime Minister Narendra Modi's strong leadership and unflinching resolve to protect the national interest in all circumstances.
Modi also said that he was convinced of the success of the forthcoming annual summit in further deepening the India-Japan special strategic and global partnership, according to the MEA.
The US leader has said he wants to pursue bilateral agreements over free trade accords to narrow trade gaps in the region - part of his "America First" clarion call.
Only host Prime Minister Prayuth Chan-ocha of Thailand and the prime ministers of Vietnam and Laos joined O'Brien and the foreign ministers sent by the other countries.
There were last-minute doubts about the deal after India raised concerns over cheaper Chinese products.
Officials said there were still final issues to be resolved by the 16 countries involved in the trade deal, which has been under negotiation for seven years.
RCEP, on the other hand, is a China-led initiative.
The decision was made during a working dinner Friday night during which "ASEAN foreign ministers chose to form a troika of three leaders only" to meet with O'Brien, according to diplomatic sources.
"Dramatically, the most vulnerable areas are exactly in Southeast Asia, in Japan, China, Bangladesh and India", he said adding that Thailand risks to have 10 per cent of its population in flooded areas by the sea. This had been a recurring theme for most South East Asian nations to prop India in regional bodies since New Delhi framed its Look East policy. The report said Beijing "exports" such practices to other countries, through its political and economic influence, undermine the conditions that have promoted stability and prosperity in the Indo-Pacific for decades.
It was in India's interest to make the Indo-Pacific concept represent something beyond security, and RCEP's economic focus provided the ideal complementarity. His remarks were consistent with the view of observers who believe in hindsight the US made a fatal miscalculation, tolerating China's human rights abuses because it thought that with a more open economy, China would open up to democratic reforms, too. The US will think this TPP is a floor, the 11 will think it is a ceiling.
"While claiming that they are committed to peaceful diplomacy, the reality is that Chinese leaders - through the PLA [People's Liberation Army] navy, law enforcement agencies, and maritime militia - continue to intimidate and bully other countries", he told a Pacific subcommittee of the U.S. Senate Foreign Relations Committee in October.
In its current form, the coalition is expected to form the "world's largest trading bloc, " Al Jazeera's Asia correspondent Scott Heidler reported from Bangkok. "Only one Asian power clearly emerged the victor from this summit", said the diplomat from an Asean member state.