Delegates from 192 countries are gathering in Copenhagen for the biggest climate summit in history. The United Nations, says the conference must deliver an ambitious, sweeping agreement, to capitalize on pledges by countries to fight global warming.

On the eve of climate summit in the Danish capital that are scheduled for two weeks, the UN climate chief said time was up to agree on the outlines of a tougher climate deal.

Yvo De Boer, Executive Secretary, UN Framework convention on Climate Change, said, "The scientific community has told us that we have a 5-10 year window of opportunity to turn the upward emission trend into a downward emission trend. That is why we have to act now."

The largest gathering for climate talks are aiming to hammer out a new agreement to curb global warming, and to replace the 1997 Kyoto Protocol.

A key issue is cutting greenhouse gas emissions. Scientists are blaming the excess amount of carbon dioxide in the earth's atmosphere, for raising average global temperatures over the past decades.

The United Nations says rich nations must accept deep cuts in their greenhouse gas emissions by 2020.

They will also seek agreement on how much financial assistance rich countries should provide, to help poor nations deal with climate change.

De Boer said the Copenhagen summit had to deliver three things.

Yvo De Boer, Executive Secretary, UN Framework convention on Climate Change, said, "And what I want to see at the end of this conference is a list of rich country targets that are ambitious, clarity on what major developing countries will do to limit the growth of their emissions and a list of financial pledges that will make it possible for the much broader developing nation community both to change the direction of their economic growth and adapt to the inevitable impacts of climate change."

UN officials say greenhouse gases must be reduced by 25 to 40 percent by 2020 below 1990's output, to keep temperatures in the less dangerous range of 2 degrees Celsius.