US President Donald Trump has decided at the last minute to extend a marathon trip to Asia, after criticism that he was expected to miss the East Asia Summit, a key meeting between Asean and eight of its partners.

"We're actually staying an extra day in the Philippines. We have a big conference, the second conference, and I think we're going to have great success," Trump told reporters at the White House before leaving for his trip on Friday.

Summit host Manila had previously said Trump would likely miss the meeting. Pundits said Trump's absence could signal a lack of interest in the forum and what it represents.

It could also raise doubts about America's credibility and influence in the Asia-Pacific, at a time when China has been drawing more of its neighbours under its wings in a challenge to US presence in the region.

The president left Joint Base Andrews, just outside Washington, bound for Hawaii. From there he will travel on to Japan, South Korea, China, Vietnam and the Philippines.  The main foreign policy item on the agenda will be efforts to contain or roll back North Korea's ballistic and nuclear missile programmes.

Trump was leaving in his wake a trail of scandal and invective that risks overshadowing efforts at top-level diplomacy.  

Before he jumped on Air Force One, Trump unleashed a Twitter barrage, pillorying his own Justice Department and calling for his former political opponents to be prosecuted.  "A lot of people are disappointed in the Justice Department, including me," Trump later said, complaining they were not adequately investigating his former Democratic rival Hillary Clinton.

His comments dealt a new blow to the traditional barriers between the executive and the judicial system, breaking the taboo over presidential interference with investigations.

Trump leaves for the Asia-Pacific region with his standing at home dramatically weakened by series of indictments against former campaign aides.  The week started with special counsel Robert Mueller – who is investigating Russian meddling in the 2016 election – indicting Trump's former campaign chairman Paul Manafort on money laundering charges.

Mueller also revealed that a Trump campaign aide had lied to the FBI about his contacts with Kremlin-connected officials, but then turned informant.

Trump may be leaving Washington, but news from the US capital is likely to trail him. New indictments are rumoured to be on the way, perhaps within days.

While he is away, Republican lawmakers will be working furiously to pass tax cuts that are make-or-break for his legislative agenda.

Trump's trip itself had already promised to be anything but easy.  It is the longest presidential tour of Asia since George H.W. Bush visited in late 1991 and early 1992.  That culminated in the 41st president fainting and vomiting at a banquet in Japan.