Specific pledges by industrial countries to cut carbon dioxide and other gases blamed for climate change is a key element of a UN treaty being negotiated by 190 nations. The talks are due to be completed at a major conference in Copenhagen before the end of the year.

But Jonathan Pershing, the deputy special envoy for climate change, said U.S. climate change legislation may not be completed by then, making it impossible for U.S. negotiators to present a final number for the Copenhagen agreement.

"We will work like crazy to get it together, and we will push enormously to have legislation," Pershing said on Wednesday. "But it does not block a deal. You can have a deal without having the legislation."

The first stage in the lengthy legislative process was completed last month when a congressional committee passed a climate bill, which must go to the full House of Representatives for approval. A parallel bill must go through several Senate committees, be passed on the Senate floor and then be reconciled with the House bill. The process could easily spill into next year, well after Copenhagen.

That means only a partial agreement might be crafted in the Danish capital, Pershing said. "It might mean that you have a framework in place as opposed to absolute numbers. Those numbers may come a bit later," he said.

"It may mean that you set all the parameters and come back six months later when there is legislation," he said in an interview during another round of U.N. talks in this German city.

Other countries have said they will make no firm commitments until they know what the U.S. will do. The European Union has pledged to cut its emissions by 20 percent below 1990 levels by 2020, but said it could increase that figure to 30 percent depending on U.S. plans.

Developing countries also are reluctant to spell out specific programs for fighting climate change without a clear understanding of the package coming from the industrial states, including financial aid.

The Copenhagen deal will succeed the 1997 Kyoto Protocol, which required 37 countries to cut carbon emissions by a total 5 percent from 1990 levels by 2012. The US rejected Kyoto, saying it was imbalanced because it made no demands on rapidly expanding developing countries. China has since overtaken the U.S. as the world's largest polluter.

In the new accord, developing countries demand that the industrial countries reduce emissions by at least 25 percent from 1990 levels by 2020. Some countries say that figure should be as high as 45 percent to avoid regular catastrophic climate events like severe drought and storms, disastrous changes in rainfall and water availability, and sea level rises threatening coastal areas.

Pershing said other crucial elements of the Copenhagen accord can be sealed that do not depend on emission reduction targets, such as financing to help poor countries adapt to climate change.

The US delegation has been lobbying to shift the focus to long-term targets rather than emphasizing a 2020 goal, it is lagging behind other countries because it did little during the eight years of the Bush administration to cut emissions.

But the EU rejects that line of reasoning, saying actions geared toward 2020 are within the life span of current leaders and governments.

By 2050 "we'll all be dead," said Artur Runge-Metzger of the European Commission.

The Obama administration has pledged to cut emissions by 17 percent from 2005 levels by 2020, and by 83 percent by mid-century. Starting now, it says, U.S. actions to limit pollution will match the EU.

Pershing indicated he thought some countries were using the issue of targets as a means to squeeze the US on other issues, which he did not specify.

"I'm not clear at all the debate is about the numbers. I think the debate is around perception, and around optics," he said.