Hosting the leader of key developing nation India at the White House a week after returning from top global polluter China, the US president said the world was "one step closer to a successful outcome in Copenhagen."

He and Prime Minister Manmohan Singh "reaffirmed that an agreement in Copenhagen should be comprehensive and cover all the issues under negotiation," Obama told a joint White House press conference as he hosted his first state visit.

"With just two weeks until the beginning of Copenhagen, it's also essential that all countries do what is necessary to reach a strong operational agreement that will confront the threat of climate change while serving as a stepping-stone to a legally binding treaty."

The December 7-18 talks already received a boost on Monday when a senior Obama administration official said the United States would announce a target for reducing its greenhouse gas emissions before the conference gets underway.

The official refused to be drawn on specific numbers but indicated the US target would not differ much from levels mentioned in legislation before Congress.

A House of Representatives bill, passed in June, calls for cutting greenhouse gas emissions by 17 percent from 2005 levels by 2020 and by 83 percent by 2050.

A slightly more ambitious bill before the Senate, but not due to be debated again until early next year, talks of a 20 percent reduction from 2005 levels by 2020.

An emissions target from the United States, the world's number two polluter and wealthiest country, was essential for the success of the conference, according to United Nations climate chief Yvo de Boer.

"The key issue here at the moment is the United States. My sense is Obama will be in a position to come to Copenhagen with a target and a financial contribution," he said in Brussels on Monday.

Obama has still not confirmed whether he will attend the conference.

Singh, also speaking before reporters at the White House, said he and Obama had agreed on "the need for a substantive and comprehensive outcome" in Copenhagen.

"Just as we partnered with each other in the shaping of the knowledge economy, we have the opportunity today to become partners in developing the green economy of the future," he added.

India, China and other developing nations have been pressing for Western nations to offer technology and other support to help them reduce the intensity of emissions blamed for global warming.

But the emerging economies have resisted legally binding requirements, saying that wealthy nations, not them, bear the historic responsibility for carbon emissions.

De Boer has ruled out the possibility that a comprehensive treaty can be reached in Copenhagen and anticipates a list of "rich country targets" and more clarity on goals from the likes of India and China.

The United States was the world's biggest carbon emitter until it was overtaken by China in 2006, according to the Global Carbon Project, a consortium of leading climate scientists.

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