World leaders and environment ministers have held marathon meetings to settle differences, as United Nations climate talks enter the last 24-hours.

Two-track talks are once again moving forward in Copenhagen. UNFCCC executive secretary Yvo de Boer said negotiation between the two working groups will form the basis for a deal. One focuses on a new climate agreement, and the other on adapting the existing Kyoto Protocol.

As major international leaders begin arriving in Copenhagen, UN Secretary General Ban Ki-Moon says he remains optimistic that a deal can be reached.

Ban Ki-Moon, UN Secretary General, said, "Unfortunately, negotiations have been going very slowly and, even at this time, negotiations are still going. This is one of the most complex, and complicated, and most difficult process, as you may imagine. But I have not seen anything that indicates we cannot seal a deal in Copenhagen. I believe we can seal a deal, still, in Copenhagen."

US Secretary of State Hillary Clinton said she would do everything possible to come to an agreement, after meeting with British Prime Minister Gordon Brown.

Hillary Clinton, US Secretary of State, said, "We're working hard. I'm here to try to do everything we can to bring the parties together, to come up with an agreement that everybody can sign off on and that's why I was meeting with the Prime Minister."

Some world leaders voiced fears the talks could fail. But the Swedish Prime Minister, who is also the European Union President, was more hopeful saying an agreement is close.

Fredrik Reinfeldt, Swedish Prime Minister, said, "We're not that far away from what we need. We will talk to the major partners that we think are needed to strike a deal to see if it is possible. That is what we are trying to achieve tonight."

Late Thursday, world leaders attended a lavish banquet at Denmark's royal palace.

UN chief Ban Ki-Moon called on them to work together to move the world back from the brink.

After the banquet, leaders returned to work to forge a political declaration with the meeting expected to continue into the early hours.

US President Barack Obama has now departed for Copenhagen, in order to sign off on a deal.

Large parts of the climate agreement fell into place on Thursday, with new offers from the US and China. But other tough issues remain unresolved.

(CCTV December 18, 2009)