Trump has been accused of turning his back on Asian allies for pulling out of a major trade pact, as fellow superpower China pursues its own deals and investment projects in the region.
Washington did not send top officials to the weekend ASEAN summit, instead dispatching commerce secretary Wilbur Ross and national security advisor Robert O'Brien.
Monday's address from O'Brien stood in contrast to earlier ASEAN meetings, which had all been attended by most heads of state.
"It's not appropriate for ASEAN to send leaders when the US representation is not on parity," one diplomat in Bangkok told AFP.
Another diplomat said: "It's not a boycott, it's just that other leaders have other meetings to attend to."
In lieu of Trump's physical presence, O'Brien read a letter from the president inviting "the leaders of ASEAN to join me in the United States for a special summit" in the first three months of next year.
In attendance was the prime minister of Thailand, which is hosting the summit, along with the leaders of Laos and Vietnam, next year's ASEAN chair.
The summit, which closes Monday, has been peppered with statements from leaders rallying against protectionism amid fears of dragging global growth made worse by the US-China trade war.
Trump, who attended the 2017 ASEAN meeting in Singapore - Vice President Mike Pence went to the one in Manila last year - could not come this year because he was busy with campaign events back home, a senior White House official said.
China's premier and the leaders of India, South Korea and Japan all made an appearance at meetings Monday, although none of them was expected at the US-ASEAN summit.
Chinese dominance
One of Trump's first moves after assuming office was to withdraw from the Trans-Pacific Partnership (TPP), a massive free-trade pact that was billed to be the world's biggest before Washington pulled the plug.
The US withdrawal "was a very strong symbolic act and the trend is continuing", said analyst Alex Holmes from Capital Economics.
"It's allowing China to advance its influence in the region," he told AFP.
The TPP has since been reborn as a watered-down version without the US, eclipsed by a China-backed agreement that is slated to be the world's biggest when finalised.
The 16-nation Regional Comprehensive Economic Partnership (RCEP) covers 30 percent of global GDP and loops in half of the world's population.
It includes all 10 ASEAN states plus China, India, Japan, South Korea, Australia and New Zealand - notably excluding the US.
But the deal faced 11th-hour stumbling blocks Monday as India failed to agree to the draft text agreed by the rest of the member. Indian Prime Minister Narendra Modi is facing fears back home from local industries that its market could be flooded by cheap Made-in-China goods.
"It is the 15 participating countries that have decided to move forward first," China's deputy foreign minister Le Yucheng told reporters, adding that the deal could be signed next year - with India if it wants to join. Australian Prime Minister Scott Morrison earlier told reporters: "It is important to have India in, and that is certainly our preference".
RCEP has gained new momentum as the US-China trade war rumbles on, with the IMF warning that global growth could slow to its lowest rate in 10 years thanks to the tit-for-tat-trade spat between the world's two biggest economies.
As negotiations were being finalised Monday, the ASEAN bloc and its allies espoused open trade in the region.
"We are once again faced with the high winds of trade protectionism," said South Korean leader Moon Jae-in.