It will be "impossible" to conclude a comprehensive climate treaty in Copenhagen in December, but a strong political deal is still a must, the UN's top climate official said Wednesday.

"It is physically impossible under any scenario to complete every detail of a treaty in Copenhagen," said Yvo de Boer, executive director of the UN Framework Convention on Climate Change (UNFCCC).

"But Copenhagen can and must agree to the political essentials that will make a long-term response to climate change clear, possible, realistic and well defined," he told journalists by phone.

The December 7-18 meeting in the Danish capital of negotiators from 192 countries "must see the end of negotiation and the beginning of technical process to work out all the details," de Boer said.

A final, five-day session of preliminary low-level talks within the UN framework will take place in Barcelona starting Monday.

In order to succeed, Copenhagen would have to bring "absolute clarity" on four key points, all of which remain essentially stalemated barely a month ahead of the December conference, de Boer said.

The meeting must decide by how much rich countries will cut greenhouse emissions by 2020 and 2050, and what major developing nations -- including China, India, Brazil, Mexico and Indonesia -- will do to limit growth of their emissions, he said.

In addition, rich countries must say how much they will give poor nations to help them reduce carbon pollution and cope with projected climate change impacts ranging from flood and drought to food scarcity and increased disease burdens.

Finally, the conference must determine how that financing -- set to run into hundreds of billions of dollars annually within a decade -- will be managed.

"We do not have another year to sit on our hands... The deal must be done in Copenhagen," de Boer said, noting that a treaty had to be signed and ratified before the end of 2012, when Kyoto Protocol provisions run out.

Once these political cornerstones are in place, "we need to finalise the details over the course of 2010," he added.

De Boer recalled that it took eight years to negotiate, sign and ratify Kyoto, which remains the only international agreement limiting the output of CO2 and other heat-trapping gases.