Environment ministers faced mounting time pressure on Thursday to shape a blueprint for a global pact on climate change with ground-breaking carbon curbs at its core.

On the penultimate day of talks in Lima, concern grew over an unwieldy draft text intended to guide negotiations which, by December 2015, must forge the most ambitious environmental accord in history.

There was also a flareup in a years-old dispute over which countries should do more to tackle carbon emissions driving dangerous climate change.

"We have been talking over the last two decades," UN Secretary General Ban Ki-moon told ministers.

"Now it's time to begin real, serious negotiations based on draft text."

He added: "We don't have a moment to lose."

"It is clear that progress over the last 10 days or so has been too slow," added European Climate Commissioner Miguel Arias Canete. "The text has grown instead of being streamlined."

Compromise "will take courage, and it will mean countries moving to the edge of their comfort zones," Canete told a roundtable late Wednesday.

The talks -- the annual forum of the UN Framework Convention on Climate Change (UNFCCC) -- are scheduled to end late Friday after a 12-day huddle in the Peruvian capital.

Their goal is to clear the way to a deal in Paris to slow the juggernaut of climate change by limiting global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels.

At its heart would be a roster of voluntary national pledges on greenhouse-gas emissions spewed by burning coal, oil and gas.

But countries remained far apart on its design.

This casts a shadow over prospects for next year, when pledges are put on the table ahead of the Paris showdown.

- Debate over 'differentiation' -

Sparring revived over "differentiation" -- how to apportion responsibility between rich and developing nations for ramping up action on carbon.

India led a chorus of developing countries on Wednesday to defend a division of responsibilities enshrined in the UNFCCC at its birth 22 years ago.

At the time, UN members were split into advanced economies, the first to burn fossil fuels to power their prosperity, and developing countries, which -- until then -- bore little blame for carbon pollution.

Europe, the United States and other advanced economies accept that some form of differentiation remains valid today.

But they argue it is senseless to shape a post-2020 deal on the basis of the world as it was in 1992.

Voracious burners of coal, developing countries now account for nearly 60 percent of global emissions, a share set to rise, they argue.

"Some are acting as if (differentiation), this basic principle of the Convention, were threatened with extinction, but this is in fact completely untrue," said Stern.

Nevertheless, "a permanent division of countries into the two categories established in 1992... makes no sense in a world with rapidly changing material conditions."

US Secretary of State John Kerry was scheduled to make a flying visit Thursday to showcase his country's efforts in fighting climate change, the American embassy in Lima said in a statement.

Negotiations on framing the post-2020 pact were scheduled to wrap up by late Thursday so that the draft can be adopted on Friday.

But the UNFCCC's meetings are notorious for textual battles and delay, in some cases prolonging closure by more than a day.

With the 2015 deadline looming, climate change has returned to the top political table following a near-fiasco at the 2009 conference in Copenhagen.

- 'Spaceship Earth' -

Scientists have pounded an ever-louder drumbeat of warming.

Right now, Earth is on track for warming this century of about four degrees Celsius, cursing future generations with species loss, rising seas, drought, floods and disease spread, they warn.

For a good chance of reaching the two degreed Celsius target, annual emissions must fall by 40 to 70 percent by 2050, and to zero or below by 2100, they say.

The head of the UN's panel of climate scientists, Rajendra Pachauri, warned Thursday that only about a thousand billion tonnes of carbon dioxide (CO2) could be emitted before the targeted limit is breached.

"We are all on spaceship Earth. We are now sharing this space of a thousand gigatonnes of CO2," he said.

"We have to divide up this space equitably and in a matter of fair and ethical considerations because otherwise we are going to exceed the 2 C."