Developing nations said they would resist "pressure and even threats" from some rich countries to step up the fight against global warming.
Talks on a broader climate pact led to an easing of tensions in public wrangling between the EU and United States, however.
About 190 nations have been meeting in Bali, Indonesia, for the past 10 days in talks set to run well into tomorrow, when the UN secretary-general will make an unscheduled return to the conference in an effort to secure a compromise.
The main negotiating bloc of developing countries, called the G77, said they were not ready to make new efforts to fight climate change by cutting emissions from fossil fuels. They fear curbs would cramp economic growth aimed at lifting millions out of poverty.
But the head of the UN climate change secretariat chief was optimistic about the Bali talks. "We're on the brink of an agreement. We're absolutely not deadlocked," Yvo de Boer said.
The G77 comprises about 150 developing countries, including China and India who are among the world's top greenhouse gas emitters.
Developing countries had come under "strong pressure" to curb their emissions of greenhouse gases, said chairman of the group Munir Akram.
"The developing countries so far have successfully resisted the kinds of pressures and even threats which we have faced to undertake commitments," he said, referring to threats of trade sanctions from unnamed developed nations.
The UN wants countries to agree by 2009 on a global climate pact to succeed the Kyoto Protocol which expires in 2012. The new pact would involve all nations, led by the United States
Kyoto binds all industrial nations except the US to cut greenhouse gas emissions by an average of 5 per cent below 1990 levels by 2012 at the latest.
Earlier today, the talks received new impetus as the European Union toned down a clash with the United States over emissions goals for rich countries.
Indonesia suggested dropping an EU-backed ambition for rich nations to cut emissions by between 25 and 40 per cent by 2020 in a bid to overcome Washington's opposition.
Mr De Boer said the new draft was the basis for a compromise because it retained a guideline, consistent with the EU's 2020 target, that world emissions should peak within 10 to 15 years and be cut by to well below half of 2000 levels by 2050.
But it was still not clear if the United States and Europe would agree to the text.
"I know that there are some concerns particularly among several countries about this quantifiable target," UN Secretary-General Ban Ki-moon said. "Somewhere down the road we must agree on that but our goal is to launch this negotiation," he said.