• World's two biggest emitters ratify global climate accord
• Ratification could push agreement into legal effect by year-end
• US, China pledge support for aviation emissions pact

The US Republican Party Platform has also questioned the legality of the executive order used to ratify the Paris deal, saying it will need the consent of the Senate before it becomes binding.

Li Shuo, a climate adviser with Greenpeace, said both China and the United States were determined to put the treaty into force as soon as possible in order to avoid the risk that any new Republican administration would reject it.

"It now looks like the Paris agreement will enter into force before the end of the year and that will really be light speed compared to almost all other international agreements," he said.

US Democratic presidential candidate Hillary Clinton is a strong supporter of the accord, but her Republican counterpart Donald Trump has dismissed man-made climate change as a hoax and says he will abandon the Paris agreement if elected.

Countries that ratify the deal will have to wait for three years after it has gone into legal force before they can begin the process of withdrawing from it, according to the agreement signed in Paris.

Ratification, however, does not mean the work is over.

Alden Meyer, international director of the Union of Concerned Scientists, said the Paris agreement's detailed rules will likely take another year or two to finalize.

"All countries will need to raise the ambition of their commitments under the agreement if we're to avoid the worst impacts of climate change and reach a goal of net zero global warming emissions by mid-century," Meyer said.

"But this is an important step forward that reinforces the US and China's continued leadership in building a robust, durable international climate framework."