* Obama and Hu agree to "significant" action on emissions
* Host Denmark welcomes Obama appeal for Copenhagen deal
* Danish PM expects U.S. to pledge firm emission cuts
* Sen. Boxer sees no consensus on 2020 US emission cuts (Adds U.S. Senator Boxer in paragraphs 6-7)
By Chris Buckley and Alister Doyle
BEIJING/COPENHAGEN, Nov 17 (Reuters) - U.S. President Barack Obama said on Tuesday next month's climate talks in Copenhagen should cut a deal with "immediate operational effect," even if its original aim of a legally binding pact is not achievable.
About 40 environment ministers meeting in Copenhagen made progress towards a scaled-down U.N. deal next month, while African leaders accepted for the first time that the December meeting would not agree a full treaty.
Obama was speaking after talks with Chinese President Hu Jintao in which he said the world's top two greenhouse gas emitters had agreed to take "significant" action to mitigate their output of carbon dioxide.
The two countries account for more than 40 percent of global carbon emissions from burning fossil fuels.
"Our aim (in Copenhagen) ... is not a partial accord or a political declaration but rather an accord that covers all of the issues in the negotiations and one that has immediate operational effect," Obama said.
But a senior U.S. senator, Obama's fellow Democrat Barbara Boxer, on Tuesday predicted that Washington officials will arrive at the Copenhagen summit without consensus on how deeply the United States can promise to cut its own carbon emissions by 2020.
Boxer, who chairs a Senate environment panel that recently approved a climate change bill that lacks enough support to pass the full Senate, left open the possibility that U.S. negotiators instead might offer a range for U.S. carbon reductions by 2020.
Denmark, host of the Dec. 7-18 climate talks, won backing from Obama and other leaders at an Asia-Pacific summit on Sunday for Danish Prime Minister Lars Lokke Rasmussen's scaled-down plan for a politically binding deal in December, and a legally binding one in 2010.
Rasmussen on Tuesday welcomed Obama's comments and said it expected the United States and all developed nations to promise firm emissions cuts and new cash to help the poor cope with global warming, even if no treaty text could be agreed.
PRESSURE ON U.S.
At the final preparatory meeting before the Copenhagen summit, environment ministers in the Danish capital put pressure on Washington to do more to unlock talks.
"My feeling is that it looks better today than when we started meeting," Danish Climate and Energy Minister Connie Hedegaard told a news conference after two-day talks.
"In the end, an agreement in Copenhagen will depend on an American number. Without a clear and ambitious number the whole agreement will be in danger," Swedish Environment Minister Anders Carlgren told Reuters.
Obama's call on Tuesday for a broad agreement taking effect immediately suggests he is keen to walk away from the climate summit with more than just a piece of paper. But the stalling of legislation hampers him.
Washington has been reluctant to promise firm emissions cuts by 2020 without domestic carbon-capping legislation which Democrat supporters hoped the Senate would approve next spring.
Andrew Light, a senior fellow at the Center for American Progress in Washington, told Reuters U.S. officials had recently raised expectations that the United States "will put down a number" in Copenhagen for carbon goals, to become part of a final deal.
The United States may also be able to offer short-term funding to help developing countries deal with global warming, he said, regarding the possible "immediate effect" of a Copenhagen deal which Obama referred to on Tuesday.
Obama said Washington and Beijing had committed to cooperate in areas including renewable energy, cleaner coal and electric vehicles.
Hu said the two sides had committed to working more closely on global warming and called for a "positive outcome" from the talks in Denmark. He has refused to back a scaled-down political deal, which Beijing says it is only "studying."
African leaders meeting in Ethiopia agreed on how much cash to demand from the rich world to compensate for the impact of climate change on the continent, but kept the figure secret ahead of next month's Copenhagen talks.
Ethiopian Prime Minister Meles Zenawi, who will represent Africa at the talks and has threatened a walkout of the African nations -- said Africa wanted a treaty in Copenhagen but could accept a "binding political agreement" as a stepping stone.
Outside the Copenhagen conference centre a group of demonstrators played dead with giant letters spelling "delay kills" -- saying Copenhagen must agree a full legal text. (Additional reporting by Richard Cowan in Washington; Writing by Charles Dick and Gerard Wynn; Editing by Dominic Evans and Eric Walsh)