BBC: he US has pledged to tackle climate change by cutting its carbon emissions 26-28% by 2025.
It made the formal offer to the UN as a step towards a global deal in Paris in December.
The EU has already promised to cut its emissions by a roughly similar proportion.
Tuesday was the deadline for wealthy nations to make their offers - but some, such as Canada, have failed to submit in time.
The announcement was made on Twitter with the words: "America is taking steps to #ActOnClimate, and the...