With the COP28 climate talks entering crunch time on Monday, U.N. climate chief Simon Stiell urged countries to come together to reach a final deal for the summit - where they were facing off over whether to call for an end to fossil fuels.

Stiell said there had been progress in resolving some disagreements over the last day, but warned that "each step back from the highest ambition will cost countless millions of lives".

Countries at the U.N. climate summit in Dubai were waiting on Monday for the COP28 Presidency, held by the United Arab Emirates, to release a new draft text of what would be the hoped-for final agreement.

"Clear the unnecessary tactical blockades out of the way. And there have been many along this journey," Stiell said at a press conference Monday morning.

He said two key issues were still in debate: how ambitious nations were willing to be in tackling climate change, and how much funding and support they would provide to back up that aim.

A coalition of more than 80 countries including the United States, the European Union and small island nations are pushing for an agreement that includes language to "phase out" fossil fuels, a feat not achieved in 30 years of the U.N. summits.

They are coming up against some strong opposition.

Negotiators and observers inside the COP28 talks told Reuters that Saudi Arabia, de facto leader of the OPEC oil producers' group, was among the main opponents of a deal to phase out fossil fuels.

Deals at U.N. climate summits must be passed by consensus among the nearly 200 countries present.

COP28 President Sultan al-Jaber has given negotiators until Tuesday to agree on what could be the world's first deal to phase out the use of fossil fuels, the main source of greenhouse gas emissions that cause global warming.

As the COP28 summit headed into its endgame, U.N. Secretary General Antontio Guterres flew back to the conference on Sunday afternoon.

"I am here to renew my urgent appeal to leaders: Recommit to the 1.5°C warming limit. End the fossil fuel age. Deliver climate justice," he said in a post on X, formerly known as Twitter.

Speaking in a gathering of ministers and negotiators on Sunday, a representative for Saudi Arabia's delegation said a COP28 deal should not pick and choose energy sources, but should instead focus on cutting emissions.

"We have raised our consistent concerns over the attempts to attack energy sources instead of emissions," the representative said.

That position echoes a call made by oil producers' club OPEC in a letter to its members earlier in the COP28 summit, seen by Reuters, which asked them to oppose any language targeting fossil fuels directly.

While greenhouse gas emissions from fossil fuels are by far the main cause of climate change, the burning of coal, oil and gas remains the world's main source of energy, powering many nations' economies.

Despite the rapid growth of renewable energy, today, fossil fuels produce around 80% of the world's energy.

Negotiators told Reuters other OPEC and OPEC+ members including Russia, Iraq and Iran have also resisted attempts to insert a fossil fuel phase-out into the COP28 deal.

Singapore's environment minister Grace Fu said on Monday that the talks had progressed in some areas, but there still was "significant" work to do.

"We have more or less narrowed down the crucial and critical issues. Having said that, there are still some gaps to finding the solution," Fu told reporters on the COP28 sidelines.