Their ongoing debate around climate change appears as an attempt by both to frame the argument, with Thunberg calling for an immediate end to fossil fuel investments in front of a packed audience less than a hour after watching Trump make his keynote address in the Swiss ski resort.

"Our house is still on fire," Thunberg said, repeating her remarks at the annual meeting of the World Economic Forum a year ago.

"Your inaction is fuelling the flames," the teenage activist added, in the latest to-and-fro with the 73-year-old president.

Trump announced the US would join an existing initiative to plant one trillion trees, but also spoke at length about the economic importance of oil and gas and called climate change activists "pessimistic" and the "heirs of yesterday's foolish fortune tellers".

Thunberg responded by referring to "empty words and promises" by world leaders.

"You say children shouldn't worry... don't be so pessimistic and then, nothing, silence."

Earlier, Thunberg called on world leaders to listen to young activists, who have followed her to Davos this year.

"I'm not a person that can complain about not being heard," she said, prompting laughter from the audience on the first day of the annual WEF meeting.

"The science and voice of young people is not the centre of the conversation, but it needs to be."

Several young activists have travelled to the Swiss ski resort of Davos this year, following in Thunberg's footsteps.

Among the "climate heroes" being celebrated by the WEF are Irish teen scientist Fionn Ferreira, who created a solution for preventing micro plastics from reaching oceans.

They also include South African climate activist Ayakha Melithafa, 17, and Canadian Autum Peltier, who has been advocating for water conservation since she was eight.

Australian Associated Press