US President Donald Trump says he will know within the first minute of his historic meeting with Kim Jong-un if the North Korean dictator is serious about denuclearisation, setting a cautious tone ahead of Tuesday's meeting between the two leaders in Singapore.

Mr Trump, who arrived in Singapore on Sunday night, told reporters at the G7 Summit in Canada before his departure that the meeting was a "one-time shot" for Mr Kim but went on to say he remained optimistic about a breakthrough.