China and India announced on Tuesday they would back the 11th-hour climate accord hammered out in Copenhagen in December, removing doubts that the world's two most populous countries fully supported the contested deal.

Ahead of a January 31 deadline, China -- the world's No. 1 greenhouse-gas emitter -- informed the UN of voluntary actions it planned to take to curb the carbon intensity of its economy.

But it did not give the accord the political endorsement of saying it sought to be "associated" with the deal, a step taken by all industrialised countries.

In a letter dated March 9, posted on the UN Framework Convention on Climate Change website, Beijing's top climate negotiator, Su Wei, instructed the UNFCCC to add China to the list of countries that support the agreement.

"I am writing to confirm that the Secretariat can proceed to include China in the list of Parties included in the chapeau of the Copenhagen Accord," he wrote in the one-sentence statement.

Inclusion in the "chapeau," or heading, of the document is equivalent to declaring an association, according to the process set down in Copenhagen.

In a parallel move on Tuesday, Indian Environment Minister Jairam Ramesh announced India would join the more than 100 other countries that have already "associated" with the accord.

"We participated in the negotiations on the Copenhagen Accord and we stand by the accord," Ramesh said.

China and the India were the last major economies to declare a fuller endorsement for the accord.

Analysts suggested that their reluctance to say they were "associated" with the agreement was based on suspicions that the Copenhagen pact could weaken the global arena, centered on the 192-nation UNFCCC process.

The Copenhagen Accord calls for limiting warming to two degrees Celsius (3.6 degrees Fahrenheit), the threshold set by many climate scientists, although critics complain the actions are only voluntary and lacking in vital details on how to achieve the goal.

It also commits rich countries to paying out around 30 billion dollars in total over the next three years, and sets a potential figure of 100 billion dollars annually by 2020, to help poor nations fight climate change.

"The Chinese are coming into line and cooling things off with the United States, which was exasperated with their attitude," commented Emmanuel Geurin, a climate policy analyst in Paris.

Last month, US climate negotiator Todd Stern said he believed that Beijing would come onboard "because the consequences of not doing so are so serious -- in a word, leaving the accord stillborn."