Washington DC - President-elect Donald Trump on Tuesday vowed to speed up environmental approvals for those investing $1 billion or more in the US.

His comments in a social media post come after an election campaign in which he promised to boost the domestic fossil fuel industry and bring down energy costs.

He has since also pledged to expand "all forms of energy production to grow our economy."

On Tuesday, Trump said any person or company investing $1 billion or more in the country "will receive fully expedited approvals and permits, including, but in no way limited to, all Environmental approvals."

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He did not provide further details on how his administration would help investors secure such approvals.

The Republican revived his "drill, baby, drill" slogan during the election campaign and in his victory speech described US oil and gas reserves as "liquid gold."

Experts have warned that Trump's presidency could slam the brakes on the green energy transition, hampering efforts to meet key climate targets.

Trump has previously pledged to tear up environmental regulations and the US' commitments to combat climate change under the landmark 2015 Paris Accords.

A US retreat from climate diplomacy could undermine global action to cut fossil fuel reliance, potentially giving other heavy polluters like China and India an excuse to scale back their own plans.