HANOI, Vietnam (AP) — President Donald Trump skipped a summit with his Southeast Asian counterparts for the third year in a row on Saturday, with rival China set to expand its influence with a massive free trade deal in the region.

National Security Adviser Robert O'Brien said Trump regretted he was unable to attend the online summit with the 10 members of the Association of Southeast Asian Nations, but stressed the importance of ties with the region.

“At this time of global crisis, the U.S.-ASEAN strategic partnership has become even more important as we work together to combat the coronavirus," O'Brien said in remarks at the opening ceremony, which was livestreamed to ASEAN members watching from their respective countries.

Trump attended the ASEAN summit in 2017, but sent only representatives during the last two meetings. A special summit with ASEAN that he was supposed to host in Las Vegas in March was called off due to the pandemic.

Trump has been busy challenging the results of the Nov. 3 presidential election, insisting he was the victim of election fraud. Most countries have acknowledged Joe Biden's victory.

The White House said in a statement that O’Brien was also representing the U.S. at an East Asia virtual summit later Saturday that included ASEAN members as well as China, Japan and South Korea. Despite Trump’s absence, it said ASEAN remains central to his vision for a “free and open Indo-Pacific," Washington's strategy to counter China’s growing influence in the region.

China’s sway in the region is set to expand with a free trade agreement that will be inked Sunday. The pact, which will cover almost a third of the world economy, includes the ASEAN nations, China, Japan, South Korea, Australia and New Zealand.

India backed out of the plan last year, and it does not include the United States, despite America’s $2 trillion in trade with the countries in the pact.

In his remarks Saturday, O'Brien touted ASEAN as the fourth-largest trading partner for the U.S., with trade reaching over $354 billion last year.

“We deeply appreciate ASEAN partners' efforts to keep the key supply chains open, factories operating and PPE flowing," O'Brien said, referring to personal protective equipment used to protect against the coronavirus.

He noted that the U.S. had contributed $87 million to combat the coronavirus in Southeast Asia, including providing American-made ventilators and PPE.

“The United States has your back and we know you have ours," O'Brien said.

At the separate summit later Saturday, China, Japan and South Korea sought deeper regional cooperation to battle the pandemic.

“Facing a possible second wave as winter is setting in, we can work together to mitigate the pandemic and to reopen the economy," said Chinese Premier Li Keqiang.

South Korean President Moon Jae-in said the region must work together toward the development and equitable supply of vaccines and treatments, and establish an effective mechanism to prepare to battle new infectious diseases.

He said the speed of global economic recovery will be very uneven amid trade protectionism and uncertainties in the financial markets. An accelerated transition to a digital economy could also widen the gap between social classes, Moon warned.

“We must work to strengthen the resilience of our economies and identify measures for inclusive, sustainable growth," he said.

Japanese Prime Minister Yoshihide Suga said Tokyo has been engaged in boosting ASEAN's economic resilience and remains committed to deepening ties with Southeast Asia.

Japan and South Korea have committed $1 million each to an ASEAN COVID-19 fund, meant to support member states in securing medical equipment and finance research on drugs and vaccines against the coronavirus.

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Ng reported from Kuala Lumpur, Malaysia. Associated Press writers Ken Moritsugu in Beijing and Mari Yamaguchi in Tokyo contributed to this report.