A binding climate change agreement might not be possible, he said at the opening of the high-level ministerial segment of the 17th Conference of the Parties (COP 17) to the United Nations Framework Convention on Climate Change.
"We must be realistic about expectations for a breakthrough in Durban. A binding agreement may be beyond our reach for now," he said.
However, there would be movement forward on key issues.
Ban urged negotiators to keep up the momentum built up last year at Cancun.
Referring to recent reports from various international agencies, he warned that the world's climate was reaching a point of no return.
"We must pull back from the abyss. You are the people to pull us back from the edge," he said.