Chinese climate negotiators raised the prospect of negotiating a legally binding climate pact at U.N. talks this weekend in South Africa, but the requirements they laid out for reaching that goal might make such a deal hard to reach.

In separate remarks to reporters and non-governmental groups, two of China’s top climate officials suggested they might participate in talks aimed at forging a new, enforceable global warming agreement by 2020. That issue, along with the question of whether industrialized countries will agree to a new set of emissions reductions under an existing 1997 climate treaty, are key stumbling blocks in the ongoing climate talks in the coastal city of Durban.

But it remains unclear how much China — which now ranks as the world’s biggest emitter of greenhouse gases linked to climate change — would commit to as part of a future international treaty.

“We do not rule out the possibility of [a] legally binding” agreement, China’s lead climate negotiator, Su Wei, said in a news conference Saturday. “It is possible for us, but it depends on the negotiations.”

In a subsequent meeting with environmental groups Sunday, Xie Zhenhua, vice chairman of China’s National Development and Reform Commission, elaborated on what his country needed before it would negotiate a binding treaty. According to several meeting participants, he laid out demands that have bedeviled negotiators for the past few years.

The list included ensuring a second commitment period of emissions targets under the 1997 Kyoto Protocol; delivery of $30 billion in climate aid to poor countries by the end of 2012 and a process for raising $100 billion annually in climate aid by 2020; the fulfillment of several programs aimed at helping developing nations cope with climate change and cut their own emissions; an international framework for reporting on greenhouse gas emissions; a scientific review to determine by 2015 whether deeper emissions cuts are needed; and a pact that recognized nations have “common but differentiated responsibilities” under any global agreement.

“China is signaling that they are trying to be flexible and constructively negotiate over the next week,” Alden Meyer, who directs strategy and policy for the advocacy group Union of Concerned Scientists, wrote in an e-mail. Meyer, who attended Sunday’s meeting with Xie, added, “Looking at Su Wei’s and Xie’s comments, China seems to be showing openness to consider a process coming out of Durban that could lead to legally binding commitments for China post-2020.”

Negotiators from several nations, however, were more skeptical about the possibility of reaching a broad compromise with China, which has taken action to slow the growth of its carbon output but has resisted the idea of enshrining these commitments as part of an international pact.

The European commissioner for climate action, Connie Hedegaard, suggested in a tweet Sunday evening that it remained unclear whether China had embraced binding climate policies on a domestic level or a global one. Europeans have called upon the nearly 200 countries that participate in U.N. climate talks to finalize a new treaty by 2015, which could take effect five years later.

“Minister Xie spoke warmly about the need for a legally binding deal,” Hedegaard tweeted. “Does that then mean that China will also be legally bound?”

Chinese government officials could not be reached for comment Sunday; State Department officials declined to comment on the matter on the grounds that the negotiations were still underway.

Jake Schmidt, international climate policy director for the advocacy group Natural Resources Defense Council, said that if Chinese officials agree to negotiate a binding treaty, it will put pressure on the Obama administration, which has argued that the details of such a pact still need to be fleshed out.

“We’ll have to see if they take the next step of committing in Durban to a mandate to negotiate a new treaty by 2015,” Schmidt, who also attended the session with Xie, wrote in an e-mail. “If they do, it sure makes it hard for the U.S. to block agreement [just] because every single aspect of their conditions weren’t met.”