A prominent US economist on Thursday said that the Trump administration's decision to exit Unesco will allow China to play a bigger role in leading global sustainability development, while Hong Kong would also be able to grasp opportunities.

The comments came after the White House earlier announced the decision to leave the UN's cultural and education agency, saying the institution's work to "advance divisive social and cultural causes" goes against its "America First" policy.

Delivering a keynote speech in Hong Kong on sustainable development, Jeffrey Sachs, an economics professor at Columbia University, said the US exit will have consequences in the coming years.

"My basic belief is that the rest of the world has to get on with it, and the rest of the world has to say, 'we go on for our planet, we expect you to return.'

"[But] I wouldn't mind if someone keeps a tally of everything the US owes that it's not paying, so that there's a bill at the end of this self-imposed isolation.

"I do believe that China has a unique role to play, and China has the capacity that the world needs for transformation," he said.

Pointing to China's massive industrial chain, he said the nation could increase exports in areas such as solar power to emerging markets and developing nations to turbocharge the global energy transition.

Separately, Sachs noted that Hong Kong will be able to contribute to regional sustainable growth by utilising its role in the Greater Bay Area.

"There are a few financial centres around the world that have the potential to play a major role in sustainable development finance, and I think Hong Kong is the best-placed of all of them," he said.

"Dubai cannot do this, Singapore can't do this, because the physical investment [for such large scale projects] is going to come from China."

The forum, organised by the Hong Kong Association of External Friendship, took place at the Convention and Exhibition Centre, attracting some 300 participants.