The nearly 200 countries gathered at the halfway point of the conference in Dubai are expected to focus on the issue of fossil fuels in the hope of reaching a consensus before the gathering's scheduled end on Tuesday.
OPEC is rallying its oil producing members to veto a proposed deal to phase out fossil fuels.
"It seems that the undue and disproportionate pressure against fossil fuels may reach a tipping point with irreversible consequences," OPEC Secretary General Haitham Al Ghais wrote in a letter to members of the group, including COP28 host the UAE.
In the letter, dated December 6, he called on them to reject any language that targeted fossil fuels in a final summit deal.
Earlier, COP28 president Sultan al-Jaber urged delegates to work hard to reach a consensus.
"Let's please get this job done," al-Jaber said ahead of the release of the draft.
"I need you to step up, and I need you to come out of your comfort zones," he added.
At least 80 countries are demanding a COP28 agreement that calls for an eventual end to the use of fossil fuels, as scientists urge ambitious action to avert the worst effects of climate change.
Such a position would be unprecedented, after three decades of UN climate summits that never addressed the future of fossil fuels head on even though they are the top source of planet-warming emissions.
The draft deal circulated on Friday includes a range of options, from agreeing to a "phase out of fossil fuels in line with best available science" to phasing out "unabated fossil fuels" to including no language on them at all.
Canadian environment minister Steven Guilbeault said he was confident the final text would include an agreement on fossil fuels.
"Even if it's not as ambitious as some would want, it will still be an historic moment."
But countries with economies that rely on fossil fuels are expected to fight hard to block a phase out.
France's climate ambassador Stephane Crouzat said the biggest obstacle was that some countries, such as Saudi Arabia, feel they can go on producing fossil fuels while cleansing their emissions using carbon capture technologies, which remain expensive and have yet to be scaled up.
"We feel it's just not realistic," Crouzat told Reuters.
Other countries said they were insisting that any fossil fuel phase-out should be led by wealthy countries as they have already been exploiting their resources for decades.
"We do not deny the impact of climate change but every country cannot be put on the same standard when it comes to the transition," Malaysian Climate Minister Nik Nazmi Nik Ahmad told Reuters on Friday.
"Being both a developing country as well as oil and gas producer, we believe that any phase-out must be gradual and fair to the developing world," he added.
Diplomats say Azerbaijan is likely to win in its bid to host the COP29 summit next year. (AP PHOTO)
The UN climate agency's chief made an impassioned plea, reminding countries that the science behind the world's goal of holding warming to within 1.5C of pre-industrial temperatures is clear.
"From the planet's perspective 1.5 is a tangible limit. It is not simply a choice," said Simon Stiell, a Grenadian citizen who is executive secretary of the United Nations Framework Convention on Climate Change.
"Two billion people will live in areas... beyond the human limit," he said.
Meanwhile, eastern European countries are working to resolve an impasse over where to hold next year's COP29 summit after Russia said it would block any European Union member as COP president.
As of Friday, diplomats said Azerbaijan was likely to win in its bid to host the event.
Bulgaria and Moldova have also offered to take on the rotating presidency.
