The UN's climate science panel bemoaned Wednesday a fresh leak of data from a landmark report on global warming that it will start releasing this year.

"Clearly, it is regrettable, all the leaked material is in draft form, internal working documents," Jean-Pascal van Ypersele, vice president of the Intergovernmental Panel on Climate Change (IPCC), told AFP.

This is the second leak in as many months from the panel's work to update its ground-breaking 2007 climate change report.

In the latest incident, blogger Donna Laframboise published thousands of pages of the draft Tuesday, denouncing what she called a lack of transparency in the process.

"These drafts are always available with the greatest transparency after the publication of a report, but to publish them beforehand just creates confusion," van Ypersele said on behalf of the panel by phone from Louvain in Belgium.

"It is not the IPCC report that was published on the web, but merely a draft that does not contain the same text as the final version."

In her blog, since republished several times on the Internet, Laframboise claimed the IPCC was attaching too much weight to the input of environmentalists rather than scientists.

The IPCC responded in a statement that the review process was open "to anyone interested in submitting comments."

"All scientific comments submitted through the review process will be considered and addressed by the authors," said the body.

Some 90,000 comments were scrutinised for the 2007 report, and even more are expected this time, van Ypersele said.

Jean Jouzel, a member of the IPCC's scientific group, said the leak was regrettable but would not hinder the panel's work.

"It would be good if these bloggers made their comments to us rather than the public," he added.

Awarded the Nobel Peace Prize in 2007, the IPCC is a favoured target for climate sceptics.

It comprises around 3,000 atmospheric scientists, oceanographers, ice specialists, economists, public-health specialists and other experts.

Their task is to give policymakers a neutral update, known as an assessment report, of the latest knowledge about climate change and its impacts.

The final draft is vetted by outside assessors and governments before it is published.

The 2007 Fourth Assessment Report had a huge impact for it declared there was scientific consensus that Earth was warming as a result of fossil fuels and that signs of climate change were already visible.

But it came under fire in the runup to the UN climate summit in Copenhagen in 2009, as climate skeptics exposed several flaws and some shoddy sourcing.

The panel was forced to apologise for claiming that the Himalayan glaciers would melt by 2035.

It had also wrongly stated that more than half of the Netherlands was below sea level, instead of 26 percent as claimed by Dutch authorities.

The Fifth Assessment Report will be published as climate negotiations aim for a new global pact on greenhouse-gas emissions by the end of 2015.

The first leak from the draft concerned Working Group 1, which gives a scientific assessment of the state of the climate. Its summary for policymakers is to be published in Stockholm this September.

The second leak concerns Working Group 2, which looks at the impacts of climate change, and whose summary is due to be issued in March 2014.

A third volume, by Working Group 3, looks at options for mitigating these impacts. Its summary is due in April 2014. A synthesis of all three volumes is scheduled for October 2014.