"I'm still optimistic, but cautious. I'm cautiously optimistic," he told AFP at the airport of the Danish capital.

"We have to wait until the end of the conference to see how serious a message COP15 is going to send," he said.

Ban said that the first week of the 12-day Conference, set to end Friday with a summit gathering some 120 world leaders, "was a good start."

The fact that "ministers and heads of state are gathering (in Copenhagen) is a good indication that some important messages would be sent to everybody," he added.

The UN chief arrived just as some 48 ministers are meeting in an informal, closed-door session to haggle over the draft text of an accord for curbing greenhouse gases that would enter into force in 2013.

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