Climate talks in Lima have entered their final day with long-running issues still dividing the parties, despite an impassioned appeal from US Secretary of State John Kerry.
Mr Kerry told the negotiators that the world was "still on a course leading to tragedy".
An ambitious deal he said was "not an option, it is an urgent necessity".
But ongoing battles are threatening to limit any progress in the Peruvian capital.
Many developed countries want to see a change in the way the nations are classified in the UN process.
Until now, the rich have been obliged to take on commitments to cut emissions while the poor have not.
Countries such as the US say that the old divisions are outdated and they want everyone to take on some form of obligation.
In his speech, Mr Kerry reinforced this idea. No country should have a "free pass" anymore, he said.
"I know this is difficult for developing nations. We have to remember that today more than half of emissions are coming from developing nations, so it is imperative that they act too."
But this approach is being resisted by a number of countries, including China and many others, who want to adhere to the idea of "common but differentiated responsibilities".
BBC News - Lima climate talks: Old divisions surface between rich and poor