The United Nations Environmental Programme (Unep) says developed countries are "stuck on weaker, conditional pledges" and that the greenhouse gas emissions targets they have set themselves are "riddled with loopholes".
COP-17, the Durban-hosted UN climate change negotiations, moved into work mode on Tuesday after the pomp and circumstance of Monday's opening ceremony.
Talks president Maite Nkoana-Mashobane, South Africa's international relations and co-operation minister, said negotiators had been urged to finalise this week all but the political issues that have to be left to the ministers and heads of state that arrive next week.
Unep said the "rather large elephant in the room" at the COP-17 was the ever-widening gap between the action needed to stem global warming and what is on the table this year.
Scientists agree that average global warming should be kept below 2° C to avoid damaging climate change.
An updated Unep Emissions Gap Report, released last week, concludes that by 2020, global emissions need to be reduced to 44 gigatonnes if the world is to keep to a "credible pathway" to limiting average global warming to below 2° C - and there is growing consensus that warming needs to be kept below 1,5° C.
Unep said it was not too late, and if "strong action" was taken, this target could be reached. This would require a focus on energy efficiency and clean, renewable energy, a halt to deforestation, improved waste management and better agricultural services. It would also require action on emissions in the shipping and aviation industries.
The global aviation industry is fighting attempts to tax its emissions, but the International Maritime Organisation's proposals to reduce emissions have been met with a softer stance from shipping companies.
Unep said in a newsletter distributed at the Durban talks that it was developed countries that needed to "raise their game dramatically". The European Union, the US and several other industrialised nations have presented different arguments and stances that all centre on ensuring that developing nations commit to legally binding targets to reduce emissions under any agreement reached this year.
Developed nations other than the US argue the 1997 Kyoto Protocol - the first commitment period of which expires next December - only covers 27% of global greenhouse gas emissions, and any second commitment period or new agreement needs to include targets for the US and main emerging economies.
The US says it will not sign any agreement that does not include China, the world's second-largest economy after the US and its highest greenhouse gas emitter (accounting for more than 20% of total emissions).
The Kyoto Protocol is the only legally binding global agreement on greenhouse gas emission reductions that contains targets for most of the industrialised world, but not the US, which refused to ratify it.