The world still has a chance to limit global warming to safer levels but time is running out, UN chief Ban Ki-moon says.

“There is still a chance to stay within the internationally-agreed ceiling” of two degrees Celsius over pre-industrial levels, Ban said on Tuesday, “but the window of opportunity is fast narrowing”.

Ban opened a high-level segment of the December 1-12 UN climate talks with ministers in Lima, bringing much-needed political muscle to the final four days of a fraught process.

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Parties remain far apart on key aspects of a deal they have vowed to ink in Paris in December next year and implement from 2020.

“I am deeply concerned that our collective action does not match our common responsibilities,” Ban told delegates.

“This is not the time for tinkering, it’s time for transformation,” he added.

Draft texts were unveiled on Monday that encapsulated a broad variety of views on how best to slash greenhouse gas emissions in the quest to curb global warming.

These documents will form the basis for political negotiations among ministers, starting Tuesday with the tough issue of climate finance.

It will be followed on Wednesday by the even thornier issue of “differentiation” – how to share out the burden between rich and poor countries.

Ministers will also hold bilateral discussions with Ban, who nailed climate change to the top of the agenda in September by hosting a special summit in New York in September.

The Lima talks have two main tasks: drafting a negotiating outline for the Paris deal and reaching agreement on the format for carbon-curbing pledges that nations are to submit from the first quarter of next year.