Donald Trump poses an "existential threat" to humanity that must be countered with "urgent and dedicated" action, Noam Chomsky has warned.

"Ridicule is not enough" to defeat the US President, said the renowned academic, who called for direct activism to combat the Republican administration's policies.

The linguist and political philosopher identified Mr Trump's stance on global warming and nuclear weapons as the two biggest threats posed by his presidency.

He told the New York Times: "The most important issues to address are the truly existential threats we face: climate change and nuclear war.

"On the former, the Republican leadership, in splendid isolation from the world, is almost unanimously dedicated to destroying the chances for decent survival; strong words, but no exaggeration."

Mr Trump last month announced the US would withdraw from the Paris Agreement on climate change, while on Thursday he said he was considering "severe things" following North Korea testing an intercontinental ballistic missile.

Mr Chomsky warned the President's vow to strengthen and expand the country's nuclear arsenal, coupled with global tensions, "poses extraordinary dangers".

He said: "In these cases, citizen action can reverse highly dangerous programmes. It can also press Washington to explore diplomatic options - which are available - instead of the near-reflexive resort to force and coercion in other areas, including North Korea and Iran."

He added: "There are in fact many ways to combat the Trump project of creating a tiny America, isolated from the world, cowering in fear behind walls while pursuing the Paul Ryan-style domestic policies that represent the most savage wing of the Republican establishment."

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Referencing Mr Trump's position on climate change, Mr Chomsky said: "The performances are so utterly absurd regarding the 'post-truth' moment that the proper response might best be ridicule."

But he added: "Ridicule is not enough. It’s necessary to address the concerns and beliefs of those who are taken in by the fraud, or who don’t recognise the nature and significance of the issues for other reasons.

"Beyond that, what is needed is action: urgent and dedicated, in the many ways that are open to us."

A leading psychiatrist warned in May that Mr Trump's mental health posed an "existential threat" to the world because of his power to launch nuclear weapons.

John Zinner said the President's inability to feel guilt made humanity “vulnerable” to his “inner rage”.