* China hails U.S. promises to push for U.N. climate pact
* OPEC fears damaging economic side-effects
BONN, Germany, March 30 (Reuters) - China and other nations hailed U.S. pledges to do more to fight global warming on Monday but OPEC dampened celebrations by predicting that a planned U.N. climate treaty would damage the economies of oil exporters.
"We welcome this positive change in attitude and approach by President (Barack) Obama and his team," China's climate ambassador, Yu Qingtai, told reporters at U.N. climate talks in Bonn attended by 175 nations.
The Obama administration made its U.N. climate debut at the start of the 11-day meeting on Sunday, promising to cut U.S. emissions of greenhouse gases by between 16 and 17 percent, or back to 1990 levels, by 2020 -- far more ambitious than goals set by former President George W. Bush.
Yu said China, which many experts say has overtaken the United States as top greenhouse gas emitter, would slow the growth of its emissions by 2020 as part of a new U.N. pact to be agreed in Copenhagen at the end of 2009.
"The U.S. is glad to be back in the negotiations. For my part I can assure the delight is mutual," Danish Climate and Energy Minister Connie Hedegaard said in a statement.
The U.S. special envoy for climate change, Todd Stern, won praise for saying Washington was strongly committed to a new climate treaty and that Washington had a "unique responsibility" as the top emitter since the Industrial Revolution.
But delegates from OPEC states, including Saudi Arabia, Qatar and Kuwait, said a new treaty threatened oil-exporting economies by promoting a shift away from fossil fuels towards renewable energy such as wind or solar power.
"The shift to a low carbon economy has a clear and deliberate outcome that will adversely impact all developing country fossil fuel exporters," said Ramiro Ramirez of the OPEC Secretariat.
He said that rich nations were urging OPEC to step up exploration and production to stimulate the world economy even as they were planning to turn their backs on oil.
OPEC nations get little sympathy from many delegations at U.N. talks since they are not viewed as among the most vulnerable -- compared to the poorest African or Asian states at risk from more droughts, floods or rising sea levels.
But a climate deal has to be agreed by unanimity, making OPEC's demands as vital as any. OPEC wants measures, for instance, to promote the capture and storage of greenhouse gas emissions from power plants, refineries and factories.
Some other developing nations said they were worried that the fight against climate change could have damaging side-effects, for instance penalising their exports by raising the costs of air freight or shipping.
The U.N. Climate Panel says that developed nations would have to cut emissions by between 25 and 40 percent below 1990 levels by 2020 to avoid the worst of climate change.
But few are planning such deep cuts. The European Union aims to cut emissions by 20 percent below 1990 levels by 2020 -- much deeper than Obama's goal of returning emissions to 1990 levels.
"It's more clear than ever that the developing countries are impatient and disappointed" that there are no targets to make deep cuts, said Harald Dovland, a Norwegian official who chairs a group looking at future commitments to fight warming.
China's Yu said that the poor had to put economic growth first and defended Beijing from criticism that it is adding to pollution by opening a coal-fired power plant every few days.
"We have the option of leaving our people in the darkness without electricity, leaving the factories idle and people unemployed, or building up the necessary infrastructure to allow our economy to grow," Yu said.
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Editing by Dominic Evans
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