US President Donald Trump has decried climate "prophets of doom" in a speech at the World Economic Forum in Davos, where sustainability is the main theme.

He called for a rejection of "predictions of the apocalypse" and said America would defend its economy.

Mr Trump did not directly name the teenage climate activist Greta Thunberg, who was in the audience.

Later, she excoriated political leaders, saying 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.

In his keynote speech, Mr Trump said that it was a time for optimism, not pessimism, in a speech that touted his administration's economic achievements and America's energy boom.

Speaking of climate activists, he said: "These alarmists always demand the same thing - absolute power to dominate, transform and control every aspect of our lives."

They were, he said, "the heirs of yesterday's foolish fortune tellers".

He also announced that the US would join an initiative to plant, restore and conserve a trillion trees.

"We're committed to conserving he majesty of God's creation and the natural beauty of our world," he said.