The expert-level gathering in Accra, Ghana kicks off Thursday, and will lay the technical groundwork for a major UN meeting in Poznan, Poland at the end of the year.
"I expect rich countries to agree on the ranges by which they feel that greenhouse gas emissions should be reduced," Yvo de Boer told AFP.
"Specific targets will probably come in Copenhagen," he said, referring to the December 2009 UN conference where the world's nations have pledged to validate a successor plan for tackling global warming after the Kyoto Protocol runs out in 2012.
Negotiations since a breakthrough in Bali last year have been stymied by a rift between rich and developing nations.
China and India have called on developed countries to lead the way in cutting CO2 emissions, while the United States and Japan say these industrialising giants must also agree to binding commitments.
The European Union stands somewhere in between, having pledged to reduce carbon pollution by at least 20 percent before 2020, compared to 1990 levels.
The UN's Intergovernmental Panel on Climate Change (IPCC) has warned that even deeper cuts worldwide may be needed to stave off potentially catastrophic impacts from global warming ranging from drought to extreme weather to rising sea levels.
Time is running out to reach an accord, said de Boer, executive secretary of the UN Framework Convention on Climate Change (UNFCCC), the forum for worldwide talks on tackling climate change and its effects.
Negotiators feel "if not an emergency, at least a sense of urgency," he said in a phone interview, noting that the UN has added four additional negotiating sessions to an already packed schedule next year.
"The Accra meeting is very important in terms of determining what instruments rich countries can use in order to achieve their targets over the longer term," de Boer said.
Separate working groups will address key components in any future climate change agreement: whether the Kyoto list of six greenhouse gases will be expanded and how they will be measured; the role of global industry-specific initiatives, favored by Tokyo and Washington; and how to integrate the impact of deforestation.
Every year more than 30 million hectares (74 million acres) of forest -- which soak up 20 percent of the carbon dioxide in the atmosphere -- are lost largely due to illegal logging and slash-and-burn agriculture.
The meeting will also focus on technical and financial assistance for developing countries, both to help cut emissions and to cope with the consequences of climate change.
"I hope that, in all these areas, countries will come to Accra with very specific proposals on the kind of language to be included in the Copenhagen agreement," de Boer said.
De Boer criticised the G8 summit goal -- unveiled last month -- of halving global warming emissions by 2050 as too vague and too distant.
"Are we talking about a binding target or an aspirational target? Who is going to be taking what share of that target? These things are not clear," he said.
The baseline against which the goal is to be measured, he added, is also unclear: Europe uses 1990, the IPCC has recommended 2000, and the Japanese prime minister spoke after the summit of 2008.
IPCC chairman Rajendra Pachauri called last month on Europe to "show the way."
It it does not, he said, "I am afraid that all attempts to manage the problem of climate change will collapse."