UNITED NATIONS - Negotiators at global climate change talks are not delivering on promises by their leaders to clinch a deal at a key meeting in Copenhagen in December, a top U.N. environmental official said on Monday.

Despite progress on some aspects of a deal to brake the rapid growth of planet-warming carbon emissions, core issues remain unresolved, said Janos Pasztor, head of U.N. Secretary-General Ban Ki-moon's climate change support team.

A U.N. climate change summit last month produced promises of action by top emitters China and the United States as well as dozens of other states, and Ban said the world was one step closer to a deal at the December 7-18 Copenhagen negotiations.

But two weeks of talks that ended in Bangkok on Friday yielded little progress on the amount of cash available to poorer nations and the size of rich nations' commitments to cut greenhouse gas emissions, Pasztor said.

There is still a disconnect between what national leaders say in summit meetings and what their negotiators offer on the negotiating floor, he told a news conference.

Pasztor noted there were now only five more negotiating days left -- in Barcelona from November 2-6 -- before the Copenhagen meeting.

Countries must maintain the positive momentum of the (September U.N.) summit and translate that into concrete proposals that can advance progress toward an agreement, he said.

Copenhagen is meant to agree on a broader framework to expand or replace the 1997 Kyoto Protocol, the U.N.'s main weapon in the fight against climate change. Kyoto, whose first phase ends in 2012, obliges 37 industrialized countries to meet binding economy-wide emissions targets between 2008-12.

Pasztor said there had been progress in Bangkok on ways to help poorer nations adapt to the effects of climate change, transfer of clean-energy technology, and reducing emissions from deforestation.

But his concerns about the deadlock on core issues were the latest gloomy assessment in the run-up to Copenhagen. European Commission President Jose Manuel Barroso said on Friday he was very worried about the talks.

At some point the leaders will have to be engaged in coming up with a solution to these issues themselves, because they are very difficult and they have impacts on the economy as a whole, Pasztor said.

(Reporting by Patrick Worsnip; Editing by Philip Barbara)