The UN's climate chief on Tuesday sounded caution over hopes that a new treaty to tackle global warming would be fully wrapped up by the end of 2009.

Yvo de Boer, executive secretary of the UN Framework Convention on Climate Change (UNFCCC), said it was possible only "the key political issues" would be nailed down by this deadline and further talks would be needed to complete the details of the accord.

"We won't see a fully elaborated, long-term agreement in Copenhagen in 2009. It won't be feasible," de Boer told a press conference here.

More than 10,000 delegates have gathered in Poznan for the December 1-12 UNFCCC meeting, which aims to advance towards a treaty taking effect from the end of 2012, when provisions expire under Kyoto Protocol.

According to the so-called Bali Roadmap, endorsed by the 192-member UNFCCC conference in Indonesia last year, the new accord should be completed in Copenhagen in December 2009.

"We should be careful not to reach too far and achieve nothing," de Boer said on Tuesday ahead of a ministerial-level phase of the talks, taking place Thursday and Friday.

"What we need to achieve in Copenhagen is clarity on the key political issues, so that everything after Copenhagen is about settling the details rather than negotiating the fundamentals," he said.

The highly technical negotiations in Poznan are mired in discord over how to share out the commitments and costs of cutting carbon pollution that stokes global warming.

Rich countries acknowledge their historical role in pushing up global temperatures.

But they say rapidly emerging economies -- including major CO2 emitters such as China and India -- must also take quantifiable action.

Developing and poorer nations argue the industrialised world should lead by example, and foot the bill for clean-energy technology and coping with global warming's inevitable impacts.

"We do have to have numbers on the table from industrialised countries otherwise the other dominoes won't fall," de Boer said.

"And it's clear that you politically also need some form of engagement by major developing countries. What form that commitment will take, what shape it will have and how it will be stated, is not clear to me at the moment".