Upon arrival in Copenhagen, Ban told The Associated Press he remains cautiously optimistic of a successful outcome, but warned that negotiators on both sides must work out their differences before world leaders arrive later this week.
"This is a time where they should exercise the leadership," Ban said. "And this is a time to stop pointing fingers, and this is a time to start looking in the mirror and offering what they can do more, both the developed and the developing countries."
Talks on a global climate deal hit a snag Monday when developing countries suspended talks amid deep distrust of the promises by industrial countries to cut greenhouse gas emissions. The but deep divisions remain between rich and poor countries over emission targets and financing for developing countries to deal with global warming.
Speaking to AP at a hotel in Copenhagen, Ban said that if negotiators cannot resolve those problems before more than 100 world leaders arrive starting Tuesday, "the outcome will be either a weak one, or there will be no agreement."
"This will be a serious mistake on the part of the negotiators and the leaders if they go back empty-handed," he said.
Ban's warning came as China and the U.S. - the world's top two carbon polluters - faced off in Copenhagen.
China accused developed countries Tuesday of backsliding on what it said were their obligations to fight climate change and warned that the climate talks had entered a critical stage.
In sharp comments made as the atmosphere at the U.N. climate conference in Copenhagen grows more divisive, Foreign Ministry spokeswoman Jiang Yu said there had been "some regression" on the part of developed countries on their position regarding financial support.
The change in their position "will hamper the Copenhagen conference," Jiang told a regular news conference in Beijing.
In Beijing's view, the U.S. and other rich nations have a heavy historical responsibility to cut emissions, and any climate deal should take into account a country's development level.
China, the world's largest polluter, is grouped with the developing nations at the talks. But the U.S. doesn't consider China one of the neediest countries when it comes to giving those nations financial aid.
"We still maintain that developed countries have the obligation to provide financial support," Jiang said, adding that it was "the key condition for the success of the Copenhagen conference."
The talks were suspended for most of Monday's session - a sign of the developing nations' deep distrust of the promises by industrial countries to cut greenhouse gas emissions.
There are only days left before the conference closes Friday, and the wrangle over emission reductions froze a timetable for government ministers to negotiate a host of complex issues. Though procedural in nature, the Africa-led suspension on Monday went to the core of suspicions by poor countries that wealthier ones were trying to soften their commitments and evade penalties for missing their targets.
Talks were halted most of the day, resuming only after conference president Connie Hedegaard of Denmark assured developing countries she was not trying to kill the Kyoto Protocol, the 1997 document that requires industrial nations to cut emissions and imposes penalties if they fail to do so. Kyoto makes no demands on developing countries.
President Obama, Chinese Prime Minister Wen Jiabao and more than 110 other world leaders are scheduled to arrive in Copenhagen in the next several days to cap two years of negotiations on an agreement to succeed Kyoto.