Beatrice Fihn, the boss of the anti-nuclear weapons group that was awarded the Nobel Peace Prize on Friday, sent a tweet describing Donald Trump as a moron just two days ago.
The tweet read: "Donald Trump is a moron."
The executive director of the Geneva-based International Campaign To Abolish Nuclear Weapons (Ican) told a news conference after the prize announcement she was trying to make a joke, "which I kind of regret now".
The joke was based on reports that US secretary of state Rex Tillerson had said the same of Mr Trump.
She added: "I think that the election of President Donald Trump has made a lot of people very uncomfortable with the fact that he alone can authorise the use of nuclear weapons."
Mr Trump has threatened North Korea with "fire and fury" over the military programme that has seen the Asian country carry out nuclear tests and develop its missile potential.