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The UN's most senior climate official said governments needed to give their negotiators “clear marching orders” to step up ambition for the remainder of Cop28.
Simon Stiell said the talks on the crucial global stocktake had led to “a starting text on the table”.
“But it’s a grab bag of wish lists and heavy on posturing," he added. "The key now is to sort the wheat from the chaff.”
Negotiators are working day and night on the text that will assess where the world stands in limiting warming to 1.5ºC and chart the way forward.
There is a pause on Thursday during which the summit site will be mostly closed. On Friday, talks will intensify amid high expectations.
The draft text currently contains wording that includes everything from a sustained "phase down" of fossil fuels and coal to taking no action whatsoever.
Mr Stiell and other officials urged countries and their negotiating teams to use much firmer language that will be presented to close to 200 parties for approval next week.
Despite the UN making it clear that the world is way off track, Mr Stiell said the stocktake negotiating text at this year's Cop is vague.
It leaves options open on the contentious issue of trying to phase down or phase out the use of fossil fuels but mainly sets out building blocks for a future outcome, rather than confirming a nailed-down plan of action.
Mr Stiell said parties needed to come up with a clear statement of intent to signal the "terminal decline of the fossil-fuel era as we know it".
“At the end of next week, we need Cop to deliver a bullet train to speed up climate action,” he said. “We currently have an old caboose train chugging over rickety tracks."
Cop28 started strongly with the early adaptation of the agenda and the launch and capitalisation of the loss and damage fund. Mr Stiell acknowledged loss and damage progress had given Cop28 “a spring in its step” but it was only the start.
As of Wednesday, about $720 million had been pledged by governments but developing nations warn at least tens, if not hundreds, of billions will be needed.
Mr Stiell called for finance to be front and centre of the talks, noting that more needs to be done after the early successes of the summit at Expo City Dubai.
“Now all governments must give their negotiators clear marching orders: we need the highest ambition, not point-scoring or lowest common-denominator politics," he said.
"Loss and damage was a win but we’re kidding ourselves if we think it’s a tick in the box for finance and support at this Cop. More is required.
“We’ve said we’ll double adaptation finance – now we have to deliver, including on the details, and set ourselves up to go much further."
He urged parties not to lose sight of the "global goal for adaptation" that was mandated under the 2015 Paris deal and aims to scale up how counties can boost their resilience to climate-related damage.
"Eight billion people are on the front lines," said Mr Stiell. "Right now, only 50 countries have national adaptation plans."
Experienced lobbyist Kelly Dent, representing World Animal Protection at Cop28, told The National Mr Stiell’s efforts to rally negotiators were “not unusual at this stage in the process”.
“These things are evolving,” she said. "Just because we get a text, that doesn’t mean it’s the end of it. These things go on and on and on."
On Wednesday, US climate envoy John Kerry said it had been "a pretty damn good week" so far but the world must "emphatically accelerate the pace of emissions reductions".
Mr Kerry spoke at Cop28 shortly after Mr Stiell made his call for negotiators to up their game.
"I'm not telling you that everybody's going to come 'kumbaya' to the table," Mr Kerry said. "But I am telling you we're going to make a best effort to get the best agreement we can to move as far as we can, as fast as we can."
The UN’s leading climate expert Prof Jim Skea said Cop28 had made considerable progress in its first week.
Prof Skea, 70, is also the current chairman of the Intergovernmental Panel on Climate Change.
The IPCC’s job is to distil the world’s knowledge and provide facts on what the science is telling us.
Prof Skea has spent his time in Dubai meeting leaders, discussing the IPCC's next review cycle and reminding people what that science says.
Asked in Dubai how the talks are shaping up, he said: “It’s a Cop that’s making progress.
“It isn’t a landmark like Paris or Glasgow but we’ve seen a lot of progress on the first day, with the loss and damage fund being established. So we definitely have seen achievements.”
