The United States will announce a target for reducing its greenhouse gas emissions before the UN climate conference in Copenhagen, removing a major obstacle to a deal, officials have said.

"Countries will need to put on the table what they are willing to do on emissions," a senior administration official told journalists. "We expect that a decision will be made in the coming days."

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 US 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.

The senior administration official said Obama could make a last-minute decision to attend the conference if "negotiations have proceeded sufficiently that going to Copenhagen would give a final impetus, a push, to the process."

As the leader of one of the world's two biggest polluters, Obama is under considerable pressure to attend the conference and show flexibility on new emission targets.

Sixty-five leaders, including from Germany, France and Brazil, have already committed to participating at the December 7-18 meeting to find a global treaty to replace the Kyoto Protocol.

United Nations climate chief Yvo de Boer told journalists in Brussels that a US emissions target was a crucial factor in the negotiations.

"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."

The two-year UN haggle leading up to the Copenhagen conference has been hamstrung by what the world's No. 2 polluter and wealthiest country will propose.

In Europe, green groups said the big question was how far the United States planned to trim its carbon output and over what timeframe, and whether this met scientific criteria for tackling global warming.

"Getting clarity from the US on what their commitment is going to be is going to be crucially important," Kim Carstensen of the World Wide Fund for Nature (WWF) told AFP.

Compared to the 1990 benchmark used by almost every other country, the expected US target only amounts to something like a four-percent reduction.

The EU has vowed to reduce its emissions by 20 percent from 1990 levels before 2020, raising the target to 30 percent in the event of an international agreement on the issue. Japan has offered 25 percent, but attached conditions.

Greenpeace urged Obama to commit to levels above and beyond what was envisaged by the US Senate bill, demanding at least a 25-percent reduction target.

"We believe that there is still room for the US to upgrade their target. We need to see more from them," Joris den Blanken, Greenpeace's EU climate and energy policy director, told AFP.

Indian Prime Minister Manmohan Singh, visiting Washington, said his emerging economy was ready to do more on climate change if developed nations made financial commitments.

"We will do more if there is global support in terms of financial resources and technology transfer."

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.

In order to limit warming to two degrees Celsius (3.6 degrees Fahrenheit), a threshold widely adopted as safe, scientists say emissions by industrialized nations must fall by 25-40 percent by 2020 over 1990 levels.