WASHINGTON - US president Donald Trump has once again targeted the young Swedish activist Greta Thunberg, retweeting comments from one of his supporters.

'Keep up the great work Kellie!' Trump tweeted as he shared the message from the anonymous account @opinion8edkellie, which had critiqued Thunberg's angry address to the UN.

'What an actress!' read the tweet. '"I should be in school." She's getting the best education socialism can steal. I won't be held hostage by someone who just got a learner's permit. Sorry kiddo! Tell Al [Gore] to try again.'

It follows Trump's earlier tweet about Thunberg, in which he wrote, mockingly: 'She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!'