Washington: US President Donald Trump's triumphant assertions about the success of the unprecedented Singapore summit are being met with scepticism and outright derision from critics seizing on the contradiction between his withdrawal from the Iran nuclear deal and his willingness to accept vague pledges from North Korean dictator Kim Jong-un.

White House officials have repeatedly stressed that this week's meeting in Singapore is the beginning, not the end, of a process that Trump's team argues could have only been jump-started with the face-to-face meeting.

The Singapore summit set out broad goals to be met in the coming months while the Iran deal, signed by US president Barack Obama in 2015 and approved by seven nations, was an imperfect end to 18 months of negotiations, they say. Criticism that Tuesday's commitment does not include specifics on denuclearisation and verification is premature, they argue.

"While I am glad the President and Kim Jong-un were able to meet, it is difficult to determine what of concrete nature has occurred," said Tennessee senator Bob Corker, the Republican chairman of the Senate Foreign Relations Committee. He said he wanted Secretary of State Mike Pompeo, who will lead the follow-on negotiations, to explain details of what the administration has in mind.