Nearly all countries broadly agree that action is needed to halt climate change, which scientists warn could put millions of people at risk by century's end. But differences persist on the shape and scope of what is to be done.

"You have gathered to launch a negotiating process that is tasked with changing the course of history," UN Secretary General Ban Ki-moon said in a videotaped address to the conference's opening in Bangkok.

A total of 164 countries are taking part in the five-day meeting, which is meant to start figuring out which country will do what after 2012, when obligations run out under the Kyoto Protocol.

The talks are the first under an agreement reached at a major conference in December in Bali, Indonesia, that called for a new treaty on global warming by the end of 2009.

UN climate chief Yvo de Boer said negotiators faced a "daunting task" balancing competing interests from each country.

The world has "considerably less than two years to craft what may well in the end be one of the most complex international agreements that history has ever seen," de Boer told reporters.

The talks come two days after tens of millions of people around the world switched off their lights for an hour in the latest global initiative to show concern about climate change.

But environmentalists accused a few developed countries of quietly trying to soften their commitment to fighting greenhouse gas emissions blamed for global warming.

Greenpeace criticised Australia, Canada and New Zealand for proposing further use of "carbon sinks" -- forests and other places that suck up emissions -- as a substitute for outright cuts in fossil fuel reliance.

"What we are beginning to see here are the emergence of the same proposals to delay and defer action that plagued the last round of negotiations," said Bill Hare, Greenpeace International's climate policy director.

The United States, the main opponent of the Kyoto Protocol, only agreed to the deal in Bali after its delegates were booed in the final session there.

Harlan Watson, the chief US negotiator at the Bangkok talks, said in Bangkok that Washington wanted a deal that was "environmentally effective, economically sustainable and consistent with sustainable development."

Japan, which is far behind in its Kyoto obligations to slash emissions to six percent below 1990 levels, has proposed moving the base year for future cuts from 1990 to 2005 -- butting heads with the European Union.

Japan's economy is steadily recovering from recession, while in the 1990s parts of the European Union were in the heavily-polluting Soviet bloc.

Watson called the Japanese proposal "an interesting idea."

"We're looking into it. There are some people who think that 1990 was advantageous to some parties," Watson told AFP.

But the UN climate chief criticised the Japanese idea, suggesting it was a distraction from talks on how far to slash greenhouse gas emissions in the future.

"To me it's like talking about a starting line of a marathon but not knowing how long the marathon is going to be," de Boer said.

Fast-growing developing economies such as China and India, meanwhile, argue that they cannot be expected to make the same types of cuts as rich countries. They also want technology from rich countries to help them curb emissions.

"Bali was a successful meeting, but only a beginning to the process," said Yu Qingtai, China's chief negotiator.

"We want this meeting to set a positive tone for the next two years," he said.

Antonio Hill, climate change policy adviser for the Oxfam aid group, said the Bangkok talks were on the surface easier than other rounds "because this is principally about process rather than substance."

"I'm very confident that this week we will walk away and everybody will be happy and laugh and say it's a route forward. The key is whether that route actually puts us on track," he said.

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