UN secretary-general Ban Ki-moon and British prime minister Gordon Brown said on Tuesday they believed a significant agreement could be reached on climate change in Copenhagen next month.
However, Ban indicated that the agreement might not be the last word on a successor to the United Nations' carbon-cutting Kyoto Protocol.
"If there is political will, I am sure that there is a way. I am reasonably optimistic that Copenhagen will be a very important milestone," Ban said after talks with Brown in London.
"At the same time, realistically speaking, we may not be able to have all the words on detailed matters," he added.
"We are going through a crucially important period. We must have a comprehensive, binding agreement in Copenhagen."
Brown, who has said he will attend the talks in Copenhagen next month, said he and Ban were determined to make progress in the Danish capital.
"We believe it is possible to get an agreement on long-term targets and on intermediate targets and at the same time to get the finance in place that will allow developing countries to know that they are being given the proper protection when they take action to deal with climate change," he said.
In a sign of the divisions to be overcome, African nations boycotted UN climate talks in Barcelona on Tuesday in a protest to urge rich countries to set deeper 2020 cuts in greenhouse gas emissions.
