The world's major greenhouse gas emitters have agreed on a roadmap that - if followed - would lead to a global pact to tackle climate change with "legal force" by 2015.

Reached after a bruising 14-day conference in the South African city of Durban, it is the first time China, the US and India have said they were prepared to make commitments to combat global warming under a single legal agreement.

The deal was struck nearly 36 hours after conference was scheduled to finish amid concerns it could collapse without a deal on whether work should start on a legal treaty.

If reached, the legal deal would not start until 2020.

Australia and other delegates at the conference welcomed the agreement as a breakthrough, but environmentalists warned it would do little to slow emissions in the short-term and left the world on a path to potentially dangerous effects of climate change.