UN Secretary General Ban Ki-moon called Friday on envoys from around the world to take a "final decision for humanity" as they battled to seal a 195-nation climate-saving pact in Paris.

"This negotiation is most complicated, most difficult, but most important for humanity. We have just very limited hours remaining," Ban told reporters as ministers grabbled with stubborn differences over pivotal issues.

The UN chief acknowledged the complexities facing sleep-deprived ministers, who were expected to miss a Friday evening deadline for a deal despite pursuing negotiations through two nights.

Ban identified several important questions to resolve.

They included how to share the burden of cutting greenhouse-gas emissions between rich and poor nations; how to finance the cost of climate change in developing countries; and how ambitious to be in curbing planetary overheating.

Possible solutions had been identified in years of negotiations leading up to the conference, he said.

Ban called on ministers to think beyond their own, national priorities.

"I am urging and appealing to all the state parties to take the final decision for humanity."