US President Donald Trump and teenage climate activist Greta Thunberg have renewed their tit-for-tat about the environment at the World Economic Forum in Davos.

Mr Trump used a speech to the forum, which has sustainability as its main theme, to decry climate “prophets of doom”.

Without actually naming Ms Thunberg – who was in the audience at the Swiss ski resort – he called for a rejection of “predictions of the apocalypse” and said the US would defend its economy.

Less than an hour later, the young Swede returned fire in her own speech, telling political leaders the world “in case you hadn’t noticed, is currently on fire”.

Environmental destruction is at the top of the agenda at the annual summit of the world’s decision-makers, which takes place at a Swiss ski resort.

“Our house is still on fire,” Ms 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 her latest to-and-fro with the 73-year-old President.

Donald Trump leaves the forum on Tuesday. Photo: Getty

Mr Trump announced the US would join an existing initiative to plant a 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”.

Ms Thunberg responded by referring to “empty words and promises” from world leaders.

“You say children shouldn’t worry … don’t be so pessimistic and then, nothing, silence.”

Earlier, Ms Thunberg called on world leaders to listen to young activists.

“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 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 advocated for water conservation since she was eight.

-with AAP