U.S. President Donald Trump has invited leaders of the Association of Southeast Asian Nations for a "special summit" in the United States in the first quarter of next year.

At the ASEAN-U.S. summit held Monday outside Bangkok, Robert O'Brien, Trump's national security adviser who attended in his stead, read a message from the president that included the invitation.

"I would also like to take this opportunity to offer an invitation to all the leaders of the ASEAN to join me in the U.S. for a special summit, meeting at a time of mutual convenience in the first quarter of 2020," the letter said.

Trump also said that the United States "remains committed to our partners and strategic partnership with ASEAN and supporting organization's central role in Indo-Pacific region and the region's political, economic and social security issues and cultural affairs."

Besides reading Trump's letter, O'Brien praised ASEAN's "bold and powerful statement" by issuing in June its outlook on the Indo Pacific, which envisions the region being one of "dialogue instead of rivalry."

(Trump's national security adviser Robert O'Brien (front))

He also used the opportunity to criticize Chinese "intimidation" in the South China Sea to try to stop ASEAN nations from exploiting their own offshore resources and block "access from $2.5 trillion of oil and gas reserves."

Claiming sovereignty over almost the entire maritime area, China has rapidly built artificial islands with military infrastructure in the South China Sea -- home to some of the world's busiest sea lanes.

"The region has no interest in a new imperial era where a big country can rule others on the theory that 'might makes right,'" he said.

Only the leaders of host Thailand, next year's ASEAN chair Vietnam and Laos, the coordinator for ASEAN-U.S. relations, attended the meeting. The remaining seven members of the bloc sent their foreign ministers in keeping with diplomatic protocol, given that O'Brien is not in Trump's Cabinet.

A diplomat quoted a U.S. message to ASEAN as expressing concern with ASEAN leaders for their "intentional effort to embarrass" Trump by not attending the meeting, which was part of the three-day ASEAN Summit.

"We are extremely concerned by the apparent decision" in relation to the Seventh ASEAN-U.S Summit, the diplomat quoted the message as saying.

"ASEAN as a whole was unhappy with U.S. President Donald Trump who decided to skip the meeting," an ASEAN source said. "They were of the view that Trump should at least send a representative who is in the Cabinet."

The source added, "Such a gesture may set a bad example for other dialogue partners in the future."

Trump's absence has sparked concern about a lack of U.S. engagement in the Indo-Pacific at a time when China is increasing its clout in the region.

Since taking office, Trump has only attended an ASEAN-U.S. summit once, in Manila in 2017, and has never attended full East Asia Summit meetings. Last year, Trump sent Vice President Mike Pence in his place.

ASEAN groups Brunei, Cambodia, Indonesia, Laos, Malaysia, Myanmar, the Philippines, Singapore, Thailand and Vietnam.

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