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 to agreement at the summit.
President Barack Obama has not yet decided whether to attend the talks.
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 momentum 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.
Domestic delays
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 constrained by delays in putting legislation to curb 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 do under the UN climate convention.
Cutting edge
In the UN climate process, targets are conventionally given in comparison with 1990 levels of emissions.
On that basis, the likely US figure amounts to a cut of just a few percent, as emissions have risen by about 15% since 1990.
This is much less than the EU's pledge of a 20% cut over the same period, or a 30% cut if there is a global deal; and much less than the 25-40% figure that developing countries are demanding.
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EU Environment Commissioner Stavros Dimas said at the weekend that the EU should pledge 30% now as a way of showing commitment.
But Saleemul Huq, a climate change specialist with the International Institute for Environment and Development (IIED) who works closely with a number of developing country governments, suggested the target would be well received as a worthy first step.
"I think it's an extremely good signal that the Obama administration is willing to put a target forward and not wait for Congress," he told BBC News.
"The targets that everyone is taking to Copenhagen are the bases for negotiation; and hopefully, the negotiations will see everyone coming up with more ambitious targets - otherwise there's no point in going to Copenhagen, we could leave it all up to the US Congress.
"The other important factor is whether President Obama is willing to go to the summit - if he does, that would be a very good sign."
The list of confirmed attendees includes UK Prime Minister Gordon Brown, French President Nicolas Sarkozy and Australia's Prime Minister Kevin Rudd.
But neither Mr Obama nor Chinese President Hu Jintao - leaders of the world's two biggest greenhouse gas emitters - are yet among them.
