US President Donald Trump and other world leaders emerged from two days of talks unable to resolve key differences on core issues such as climate change and globalisation, slapping an exclamation point on a divisive summit that left other nations fearing for the future of global alliances in the Trump era.

The scale of disharmony was remarkable for the G20. Even as negotiators made a good-faith effort to bargain towards consensus, European leaders said that a chasm has opened between the US and the rest of the world. "Our world has never been so divided," French President Emmanuel Macron said as the talks broke up.

The divisions were most bitter on climate change, where 19 leaders formed a unified front against Trump. But even in areas of nominal compromise, such as trade, top European leaders said they have little faith that an agreement forged today could hold tomorrow.

Macron said world leaders found common ground on terrorism but were otherwise split on numerous important topics.

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Without directly referring to Trump, Macron said: "I will not concede anything in the direction of those who are pushing against multilateralism. We need better coordination, more coordination. We need those organisations that were created out of World War II. Otherwise, we will be moving back toward narrow-minded nationalism."

German Chancellor Angela Merkel, who hosted the summit in Hamburg, said: "Wherever there is no consensus that can be achieved, disagreement has to be made clear. Unfortunately - and I deplore this - the [US] left the climate agreement. I am gratified to note that the other 19 members of the G20 feel the Paris agreement is irreversible."