Negotiators from up to 180 countries began gathering here on Sunday for talks aimed at reaching the most ambitious treaty yet for sparing the Earth from the worst ravages of global warming.

The five-day talks, starting Monday, follow marathon negotiations in December on the Indonesian island of Bali where the world set a 2009 deadline for thrashing out a landmark pact to battle climate change.

The Bangkok meeting is the first step toward reaching that new agreement, which should take effect when commitments on cutting harmful greenhouse gas emissions under the existing Kyoto Protocol expire in 2012.

Even the United States, which pulled out of the Kyoto deal, is taking part despite its reputation as a naysayer in efforts to cut emissions of greenhouse gases such as carbon dioxide, which trap the sun's heat and warm the planet.

The talks "are critical in the sense that the conference in Bali last year formally agreed to launch negotiations, which have to be concluded at the end of 2009," said Yvo de Boer, head of the UN climate body tasked with hammering out the treaty.

"I don't expect many sticking points at this meeting. What this meeting has to do is agree a work programme and agree what is going to be discussed so that we know that we can meet the deadline in a year-and-a-half's time," de Boer told AFP.

He urged countries to stay focused on the task at hand, and not get bogged down in the kind of details that almost derailed the Bali talks.

"If you look at the amount of time available in Bangkok ... there is an awful amount of work to be done in very little time," he cautioned.

Talks in Bali almost fell apart as nations fought over who was historically responsible for climate change, who should foot the bill, and whether both rich and poor nations should have binding targets on cutting carbon emissions.

Europe and developing countries want rich nations to set a binding target for 2020, requiring them to slash greenhouse gas emissions to 25 to 40 percent below their levels in 1990.

Under US pressure the final Bali Roadmap did not include explicit goals. Frustration with the US stance grew so great in Bali that American delegates were booed during the conference's closing hours.

However, with the US presidential elections later this year, President George W. Bush's administration may not want to leave the White House with a legacy as holdouts against environmental progress, activists said.

"There is a kind of a legacy issue at play here for the Bush administration, I think they want to be viewed as constructive in its last year," said Angela Anderson, director of the global warming programme at the Washington-based Pew Environment Group.

No one expects a major breakthrough at the Bangkok talks, which are designed to allow countries to stake their starting positions in negotiations that will continue through next year.

"Every country comes at this now trying to figure out what's in their individual interests as well as the global interests," said Anderson.

But activists around the world have kept up the pressure by keeping the issue in the spotlight, sometimes by turning the spotlight off.

At least 26 cities across the globe joined an "Earth Hour" campaign on Saturday evening, dimming their lights for one hour to demonstrate how the planet can save energy.

The human risks of climate change were also highlighted Friday when the UN Human Rights Council passed a resolution declaring the problem a human rights issue, noting that the poor are more vulnerable to the effects of global warming.

Global scientists last year delivered their starkest warning yet -- that without action, global warming could have an irreversible impact on the world, bringing hunger, floods, drought and the extinction of many plants and animals.

The UN Framework Convention on Climate Change (UNFCCC), which is holding the Bangkok talks, has 192 member nations.