President Donald Trump mocked teenage activist Greta Thunberg after she gave an impassioned speech at the United Nations urging world leaders to do more to combat climate change.

The Swedish 16-year-old activist scolded the audience at the UN Climate Action Summit on Monday, repeatedly asking, "How dare you?"

Appearing close to tears during the speech, Ms Thunberg said: "We are in the beginning of a mass extinction and yet all you can talk about is money. You are failing us."

Mr Trump tweeted a video of the speech late on Monday night and said: "She seems like a very happy young girl looking forward to a bright and wonderful future. So nice to see!"

Trump had spent only a few minutes at the climate summit, a centerpiece of this year's UN General Assembly schedule.