* China willing to discuss global 2050 emissions goal
* U.S., China officials aim for Copenhagen deal
By David Fogarty and Richard Cowan
COPENHAGEN, Dec 9 (Reuters) - China said on Wednesday it wanted the United States to offer sharper carbon cuts by 2020, and President Barack Obama's top aides promised "robust" engagement with world leaders at Copenhagen climate talks.
Success at the Dec. 7-18 climate talks in Copenhagen, meant to agree the outline of a new treaty, will hinge on agreement between the United States and China which together emit 40 percent of global carbon dioxide.
Beijing's top climate envoy Xie Zhenhua said the world's biggest emitter wanted to play a constructive role.
Xie urged the United States to increase its planned cuts in carbon emissions by 2020 and said China could "discuss" a target to halve global greenhouse gas emissions by 2050 if developed nations sharpened carbon cuts and raised financial help.
"If the demands of developing countries can be satisfied I think we can discuss an emissions target" to halve global emissions by 2050, he said.
Developed nations want the world to agree a global halving of emissions to indicate that all are willing to take part in the climate change fight. Many developing nations say the rich have to commit to more first.
"I do hope that President Obama can bring a concrete contribution to Copenhagen," Xie told Reuters. When asked whether that meant more than Obama's existing proposal, a 3 percent cut of emissions by 2020 on 1990 levels, he said "Yes".
The deputy chairman of the powerful economic planning superministry, the National Development and Reform Commission (NDRC), rejected a U.N. proposal for $10 billion per year financial aid for poorer nations from 2010-2012 as "not enough."
He initially told Reuters rich countries should make emissions cuts of 25-40 percent versus 1990 levels by 2020, but clarified later that China was sticking to its past insistence of cuts of "at least 40 percent".
U.S. officials vowed support for a deal in Copenhagen.
"We are seeking robust engagement with all of our partners around the world," U.S. Environmental Protection Agency Administrator Lisa Jackson said at a press conference.
Speaking just days after her agency announced that it intends for the first time to regulate carbon dioxide emissions, Jackson declared: "We are seeking to prevent the rapid approach of climate change."
Britain, Australia, Mexico and Norway were preparing to outline ideas in a document for raising billions of dollars towards 2020, aimed at breaking a deadlock on climate finance.
"I can confirm that there will be an initiative from these countries, not on the size of the amount but about how we see the structure of finance in a new agreement," said Hanne Bjurstroem, head of the Norwegian delegation.
A British official said the document would look at ideas for a green fund for developing nations, partly to help them adapt to climate changes such as floods, droughts, rising sea levels and species extinctions.
Some developing countries resisted a lower ambition for the Copenhagen conference, which most nations now accept should agree a tough deal but that time has run out for a legally binding treaty -- to be sealed next year.
Connie Hedegaard, Denmark's minister to the conference, said she would consult on a demand by low-lying nation Tuvalu for a legally-binding outcome from the Copenhagen talks.
China's Xie said he wanted a legally binding outcome in Copenhagen, but added that if that were no longer possible a final deal in June 2010 "would be very good".
Some developing countries continued to complain that they had been by-passed by a Danish draft text for a final declaration, circulated among a select group of countries. "The Danish text is an extremely dangerous document," said Lumumba Stanislaus Di-Aping of Sudan, a spokesman for the G77 plus China group of developing nations.
"How many countries have participated in this document? Are there just 20 to 30 countries who will decide for the whole of humanity?" said Bolivian delegate Angelica Navarro Llanos.
Hedegaard reiterated on Danish TV 2 that the document was not a proposal from the official Danish hosts of the talks.
(Additional reporting by Richard Cowan, David Fogarty, Emma Graham-Harrison, John Acher and Sunanda Creagh in Copenhagen; Writing by Gerard Wynn; Editing by Dominic Evans) For Reuters latest environment blogs, click on: blogs.reuters.com/environment/