US draft legislation is being re-visited to take account of coal interests
The target is expected to be in line with figures contained in legislation before the Senate - a reduction of about 17-20% from 2005 levels by 2020.
The absence of a US target has widely been seen as the single biggest obstacle（障礙） to agreement at the summit.
At the weekend, the hosts of the Copenhagen conference announced that more than 60 heads of state and government had pledged to take part in the two-week negotiating session.
Mr Obama will join them if it appears that his presence would increase chances of the 192 parties reaching agreement, the official indicated.
"There's been recognition that if we want to keep momentum3（動力，要素） going, numbers have to be put on the table," said Peter Bahouth, executive director of the US Climate Action Network, a network of organisations lobbying for action on the issue.
"There's been pressure for the US to come (to Copenhagen) with its hands full rather than empty, and I think what we're seeing are the results of that."
In the last week, Mr Obama has discussed climate change with a number of other world leaders including Indian Prime Minister Manmohan Singh, President Hu Jintao of China and Japan's Prime Minister Yukio Hatoyama.
Although Mr Obama campaigned on a promise to cut emissions, and pledged global leadership on climate change on assuming office, the US position has been constrained4（被強迫的，拘泥的） by delays in putting legislation to curb5（抑制，束縛） greenhouse gas emissions through Congress.
The House of Representatives passed a bill in June that would cap emissions and establish a national carbon trading scheme.
But progress of a similar bill through the Senate is not likely before March at the earliest.
Administration officials have indicated that the targets are being discussed with senior senators in an attempt to ensure that the Senate will back whatever target Mr Obama takes to Copenhagen.
It is not clear when the target will emerge, but there are now less than two weeks before the summit opens on 7 December.
There will also be pressure internationally for the US to say how much money it is prepared to transfer to poorer countries to help them fight climate change, as it is bound to（必然，不得不） do under the UN climate convention（大會，協定）.
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