A trust fund set up to help a South Pacific nation gravely threatened by climate change has invested in coal mining, gas exploration and the world's largest crude oil refinery, an AFP investigation has revealed.

Low-lying Pacific island nation Tuvalu said it was reviewing the "fossil fuel exposure" of the $200 million fund after it was presented with AFP's findings.

Few countries are more exposed to climate change than Tuvalu, a chain of coral atolls reckoning with acidifying oceans, tropical disease and rising seas.

Land is already so scarce across the archipelago halfway between Australia and Hawaii that the international airport runway doubles as a makeshift sports field.

With a fragile economy and few natural resources, Tuvalu relies on a government trust fund to help foot the spiralling costs of the climate crisis.

Tuvalu has entrusted management of its single-largest financial asset to advisory firm Mercer, which has invested in funds holding stakes in major fossil fuel companies, according to financial records and government reports reviewed by AFP.

Tuvalu climate activist Richard Gokrun said it was "really shocking" to see the nation tied up with fossil fuel companies.