Delegates at 175-nation U.N. talks in Bonn on ways to step up the fight against global warming are locked in a semantic dispute -- but a vital one which will determine how ambitious a new deal is and how far it can be enforced in international law.

"It certainly has big legal implications," Yvo de Boer, head of the U.N. Climate Change Secretariat, told Reuters Tuesday. He said he speaks broadly of a "deal," "agreement" or "pact."

More than 190 nations launched a two-year push in 2007 in Bali, Indonesia, for what was described as an "agreed outcome" to fight global warming to be produced at a meeting in Copenhagen in December 2009.

The idea of a new "protocol" or "treaty," favored by many developed nations, worries many poor nations since the words imply a legally binding deal backed by sanctions for non-compliance.

But a non-binding "decision" in Copenhagen alarms many developed nations who want developing countries to take on tougher commitments to avert projected increases in heat waves, droughts, floods and rising seas.

"It's impossible to say what the end result will be," de Boer said. The March 29-April 8 meeting in Bonn is looking at issues including the extent of curbs on greenhouse gas emissions needed by 2020.

All developed nations except the United States already have binding commitments to cut emissions of greenhouse gases, mainly from burning fossil fuels, by an average of 5 percent below 1990 levels by 2008-12 under the Kyoto Protocol.

Many are far over target but de Boer said most Kyoto countries looked capable of reaching the goals with their planned measures to curb emissions. And recession is curbing use of fossil fuels.

Kyoto countries have already agreed to make deeper curbs beyond 2012 and many want developing countries, such as China and India, to take on legally binding commitments. China, the United States, Russia and India are the leading emitters.

Australia, for instance, has outlined two options for what it calls a "post-2012 treaty" for all nations. Most poor nations favor non-binding goals for themselves.

President Barack Obama has said the United States will cap emissions in what the Washington generally refers to as an "agreement."

"But some say 'what's the meaning of legally binding?'" de Boer added. "Is someone going to arrest (U.S.) President Barack Obama if he doesn't reach his target?"

Under Kyoto, countries that fail to make the agreed cuts will have to make extra cuts in a planned new period.

Former President George W. Bush kept the United States out of Kyoto, saying it wrongly omitted goals for poor nations and would damage the U.S. economy.

Environmentalists fear that the global economic crisis will deflect attention from efforts to fight climate change.