THE HAGUE, July 23 (Reuters) - The United Nations' highest court on Wednesday told wealthy countries they must comply with their international commitments to curb pollution or risk having to pay compensation to nations hard hit by climate change.

In an opinion hailed by small island states and environmental groups as a legal stepping stone to make big polluters accountable, the International Court of Justice said countries must address the "urgent and existential threat" of climate change.

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While the decision was stronger than most expected, its impact may be limited by the fact that the United States, the world's biggest historical greenhouse gas emitter, and second biggest current emitter behind China, has moved under President Donald Trump to undo all climate regulations.

With scepticism over climate change spreading in the U.S. and elsewhere, Judge Iwasawa laid out the cause of the problem and the need for a collective response in his two-hour reading of the court's opinion.

"Greenhouse gas emissions are unequivocally caused by human activities which are not territorially limited," he said.