A working lunch scheduled to take place after the leaders' discussions appeared to be cancelled and Trump's press conference was moved ahead by two hours, according to the White House.

It was not clear whether the leaders would still hold a signing ceremony, originally set to take place after lunch and before Trump's press briefing.

White House spokeswoman Sarah Sanders confirmed only that "there has been a programme change", without elaborating.

"The negotiations are ongoing, we expect that to wrap up in the next half hour," she said as Kim and Trump remained behind closed doors.

A table set for the working lunch remained empty and a live feed of the venue was suddenly cut before the White House announced the programme change.

The last-minute confusion raised questions about what if any concrete progress was made at the Hanoi summit, the leaders' first face-to-face since their historic meeting in June.

Trump may also be eager to return to Washington after his longtime lawyer and former fixer Michael Cohen testified before Congress, saying his ex-boss was a liar who engaged in possibly illegal conduct while in office.

"Today, I am here to tell the truth about Mr Trump," Cohen told the nationally-televised hearing.

"He is a racist. He is a conman. He is a cheat," who inflated his assets when it suited him "and deflated his assets to reduce his real estate taxes", Cohen said.