The 12-day Durban conference is eyeing a deal which secures the future of the Kyoto Protocol, set to be cast into limbo just a year from now.
It also would seek to coax China, the United States, India and Brazil into a new treaty on emissions curbs.
Wendel Trio of Greenpeace said everything now depended on four days of talks among environment ministers, due to start on Tuesday.
"There is progress on some of the smaller technical issues, but the big political questions will have to wait till ministers arrive," he said.
The conference's high-level session begins on Tuesday afternoon.
At the heart of the manoeuvring is the future of Kyoto, which stipulates legally-binding targeted curbs in greenhouse gases for rich countries.
The European Union (EU) -- nearly alone -- has offered to extend its Kyoto pledges after they expire at the end of 2012.
But it will do so, it says, only if major emitters -- including the United States and emerging giants such as China -- back plans for a new binding pact that would be completed by 2015 and take effect by 2020, when the current roster of voluntary pledges runs out.
This would achieve what the nearly disastrous Copenhagen Summit of 2009 failed to do: forge a worldwide treaty binding all the big parties to emissions constraints.
After a week of mixed signals, China declared on Sunday that it could envisage post-2020 binding commitments -- provided a range of conditions, including the survival of Kyoto, were met.
"I don't know if this is a shift in China's position, but it is clearly a signal that China intends to be flexible and constructively negotiate over the next week," said Alden Meyer, a veteran observer with the Union of Concerned Scientists, a Washington NGO.
If so, that would set the scene for a game of climate poker between the top two carbon polluters.
For now, the United States has resisted the EU roadmap, saying it cannot sign up to a legal framework the contents of which have not been spelt out.
Domestic politics weighs heavily in this thinking. There are presidential elections less than a year away, with the risk of a backlash from a bailout-weary public worried about the cost to their wallets of a climate treaty.
But Meyer said that this obstacle could be overcome if -- in the next few days -- the US prodded the Chinese into being clearer and more ambitious in what they were offering.
"If the US delegation could come home with an agreement where they could legitimately say that China was willing to negotiate post-2020 legally binding commitments for China, that would be a huge accomplishment for the US," he said.
Whether this may happen, though, is far from clear.
The US reaction on Sunday was muted. A senior US official acknowledged he had not seen the details of the Chinese proposal but was initially skeptical that it contained anything new.
"I don't think that there is anything different," he said. "The question is, what are they prepared to agree to here in Durban," said the official, speaking on condition on anonymity.
The other big question is this: could India, and also Brazil, be drawn into the roadmap-plus-Kyoto deal?
J. M. Muskar, the head of the Indian delegation, took a strongly conservative line.
"Our objective in these negotiations should not be to launch a new process for a new climate treaty, but (...) enhance implementation of the principles and provisions of the existing and valid climate treaty," he told reporters, referring to the UN Framework Convention on Climate Change (UNFCCC) and the so-called Bali Roadmap, laid down in 2007.