Delegates from more than 100 countries convened in Brussels this week to discuss the report and have yet to agree on all its contents, less than 24 hours before its scheduled Friday release, people familiar with the talks said.

It predicts rising temperatures will lead to more hunger in Africa, the melting of Himalayan glaciers, more heatwaves in the United States and damage to Australia's Great Barrier Reef.

"There is wrangling happening," said Hans Verolme, director of the global climate change programme at WWF, an environmental group that is an observer to the meeting.

"There are some who are questioning the scientific basis ... of some of the summary statements, which is leading the authors to have to go back to the underlying document."

The UN panel's report is the most authoritative study since 2001 on the regional impact of climate change.

Verolme said the fact world leaders would read the report's summary had added pressure for consensus on the wording.

"There is discussion whether something is 'likely' or 'very likely', and my sense is that is because people are aware here that heads of state are paying attention," he said.

"If the text says this is very likely, the response (from governments) has to be very significant."

The Intergovernmental Panel on Climate Change (IPCC) draws on work by 2 500 scientists. A previous report released in Paris in February concluded it was more than 90 percent likely that recent warming had a predominantly human cause.

"They know they are under the gun, but it could run late," one delegate said of the Brussels meeting. "It's more complex than it was in Paris and they are further behind schedule."

The IPCC has only once broken up without a deal, at talks in Geneva in 1995. It met successfully in Montreal a few weeks later. "It's not the end of the world if you have to give it a pause," said James Bruce, a Canadian who chaired those talks.

Environmental groups said this week governments must act to reduce greenhouse gas emissions or risk exacerbating poverty in developing countries and destroying natural wonders worldwide.

The report says rising temperatures will have costs for society even though some countries, such as Canada and Russia in the north, might benefit for a while from higher farm yields.

"Impacts of unmitigated climate change will vary regionally but, aggregated and discounted to the present, they are very likely to impose costs ... and these costs would increase over time," a draft copy says.

The draft says "roughly 20-30 percent of species are likely to be at risk of irreversible extinction" if the global average temperature rises by 1,5-2,5 degrees Celsius (2,7-4,5 degrees Fahrenheit).

Seas could keep rising for centuries and the report emphasises the link between human activities and climate change.

"At the global scale the anthropogenic (human) component of warming over the last three decades has had a discernible influence on many physical and biological systems," it says.

It says there is "medium confidence" that Greenland and West Antarctic ice sheets could start melting if temperatures rise more than 1-2 degrees Celsius "causing sea level rise of 4-6 metres over centuries to millennia".