China is emerging as the potential deal-maker or breaker at this year's UN climate talks in Durban, South Africa.
China's delegates have said they are willing in principle to take part in a future, legally binding deal provided key conditions are met.
But the BBC's environment correspondent says several critical details of its position remain unclear.
The Chinese position is critical to achieving an outcome that everyone can live with.
The EU and many of the smaller and poorer developing nations want talks to begin soon on a new global deal that should be legally binding and include all countries.
In return, the EU is prepared to put its next round of emission cuts - 20% from 1990 levels by 2020 - under the umbrella of the Kyoto Protocol.
Other nations such as Norway, Switzerland and Australia would be likely to follow the EU's lead, though others - notably Japan, Russia and Canada - will not.
China is willing to agree to talks on a new legally-binding deal provided five key conditions are satisfied - among them, that the EU and others remain within the Kyoto Protocol.
But what is meant by "others" has not been spelled out. If it includes Canada, Russia and Japan, the BBC's correspondent says a deal appears extremely unlikely.
National development and reform commission vice chair Xie Zhenhua told reporters that "after 2020, what we need to negotiate should be a legally binding one, or some document to that effect".
But it also not clear how much negotiation China would be prepared to entertain.
Several delegates have said that the flexibility China is propounding in public is not reflected in behind-the-scenes negotiations.
China is emerging as the key deal-maker or deal-breaker as the talks head into ministerial discussions.