BANGKOK — Asian countries held conclusive talks on what could be the world’s biggest trade pact and there will be an announcement of success at a summit in Bangkok, despite doubts raised by India, the Thai hosts said on Monday.

But China, a champion of the Regional Comprehensive Economic Partnership (RCEP), said 15 members had agreed to move ahead without India, while leaving the door open for it to join a deal that has been given new impetus by the United States-China trade war.

Despite a message of support from US President Donald Trump to the Association of Southeast Asian Nations (ASEAN), regional countries noted that Washington had downgraded its delegation for the annual Asian gathering.

Southeast Asian countries hoped to announce at least provisional agreement on the 16-nation trade bloc, which would account for a third of global gross domestic product and nearly half the world’s population.

But demands raised recently by India meant negotiations among ministers went late into the night. The bloc includes the 10 ASEAN members plus China, South Korea, Japan, India, Australia and New Zealand.

Thai Commerce Minister Jurin Laksanawisit told reporters that the 16 countries had reached agreement and would make a statement later on Monday — while acknowledging that some details still needed to be sorted out.

He said the plan was to sign the deal next year, under Vietnam’s chairmanship of ASEAN.

But Chinese Vice-Foreign Minister Le Yucheng said 15 countries had decided to move ahead first.

“The text based negotiations have been completed and issues of market access have been essentially concluded,” he said.

“Whenever India is ready they are welcome to come on board.”

An Indian official with close knowledge of talks said not everything had been resolved and discussions were still going on.

New impetus to complete the deal has come from the Sino-US trade war, which has knocked regional growth, but India fears a potential flood of Chinese imports and officials with knowledge of the negotiations said India had raised late demands.

One advantage for Southeast Asian countries of including relative heavyweight India in the trade pact would be less domination by China.

Diplomatic and security calculations in Southeast Asia have shifted under the Trump administration.

And the US decision to send a lower level delegation to the back-to-back East Asian Summit and US-ASEAN Summit this year has raised regional concerns that it can no longer be relied on as a counterweight to China’s increasing might.

Because of the downgrade in the US delegation, officials from only three of the 10 regional countries joined the usual US-ASEAN meeting.

US Commerce Secretary Wilbur Ross told a business meeting on the sidelines of the summit that the administration of US President Donald Trump was “extremely engaged and fully committed” to the region.

White House national security adviser Robert O’Brien brought a personal message from Mr. Trump offering to host a meeting of Southeast Asian leaders in the United States.

He also condemned Chinese “intimidation” in the South China Sea, where several regional states reject China’s sweeping maritime claims and complain that Beijing is illegally stopping them from exploiting their energy resources and fishing grounds.

But diplomats and analysts said the message from Washington was clear.

“Doubts have been raised in a more serious way about the Trump administration engaging and this may also play into the hands of other superpowers in pushing their own agendas,” said Panitan Wattanayagorn, a former Thai national security adviser.

In a statement as this year’s ASEAN chairman on the group’s 35th summit, Thailand said leaders of the region “welcomed the conclusion of the Regional Comprehensive Economic Partnership (RCEP) negotiations and the commitment to sign the RCEP Agreement in 2020,” adding that this development “will significantly contribute to an open, inclusive and rules-based international trading system and expansion of value chains.”

On the South China Sea, the group “reaffirmed the importance of the full and effective implementation of the 2002 Declaration on the Conduct of Parties in the South China Sea in its entirety” and noted “progress of the substantive negotiations towards the early conclusion of an effective and substantive Code of Conduct (CoC) in the South China Sea within a mutually agreed timeline,” citing “completion of the first reading of the Single Draft CoC Negotiating Text in July 2019.”

Beijing had proposed a three-year timetable to conclude CoC talks.

At the same time — though without naming China — the group also “discussed the matters relating to the South China Sea and took note of some concerns on the land reclamations and activities in the area which have eroded trust and confidence, increased tensions and may undermine peace, security and stability in the region.”

Despite the maritime row, ASEAN and Beijing have been working on various avenues for cooperation.

At the end of the 22nd ASEAN-China Summit in Bangkok on Nov. 3, a separate statement by Thailand as chair of the group said that the ASEAN and China have agreed to develop a new Plan of Action for stronger cooperation for on peace and prosperity for 2021-2025.

“We looked forward to the development of a new Plan of Action (2021-2025) as a guiding document to further strengthen ASEAN-China cooperation in areas of mutual interest,” the statement read in part. — Reuters with reports from C. A. Tadalan and G. M. Cortez