More than 900 delegates flew into Bangkok this week for a UN-led meeting on global warming, spewing about 4,181 tonnes of carbon dioxide into the atmosphere, an official from the United Nations climate body estimated.

Few would argue against holding such talks, which are vital to crafting a new pact on battling climate change, but activists are urging the world to include air and sea travel in any new accord.

"Aviation and maritime shipping are very big sources of emissions and they're growing fast," said David Doniger, climate policy chief at the New York-based Natural Resources Defense Council.

"I think that one thing that's not acceptable is to leave those sectors uncontrolled on the theory that they don't belong to anybody," he added.

Industry and green groups estimate that air travel accounts for between two and four percent of the world's emissions of greenhouses gases such as carbon dioxide, which trap the sun's heat and cause temperatures to rise.

Emissions from the sector look set to rise, however, with the number of global travellers predicted to double by 2020.

International aviation and shipping were excluded from greenhouse gas emissions reductions targets laid out in the 1997 Kyoto Protocol, the current global treaty addressing climate change.

Delegates from more than 160 countries are now in Bangkok trying to thrash out a work plan for a new agreement on how to curb emissions when the Kyoto Protocol's deadlines run out in 2012.

"I think everybody agrees that we have to find some way of addressing emissions resulting from aviation and shipping," said Yvo de Boer, the UN's climate chief.

"The big question is how. Is that inside the convention process or is it outside the convention process?"

Bill Hare, climate policy director with Greenpeace, said that airline emissions were tricky given that many countries are involved in one flight -- the departure point, arrival point and the nationality of the operator.

He argues that aviation emissions should be included in binding greenhouse-gas cuts for rich nations expected to be laid out in the new pact, with the country selling the fuel taking responsibility for the emissions.

"If you want to be an aviation hub, then there is a carbon cost to it," he told AFP.

This would then spur industry into developing more energy-efficient engines and air craft, he said.

Under the Kyoto agreement, UN organisations the International Civil Aviation Organization (ICAO) and the International Maritime Organization (IMO) were tasked with coming up with a solution to the industry's climate footprint.

"The bottom line is they haven't done anything," Hare said.

Some nations are pushing to leave the issue with the UN bodies, while the European Union has talked about a sector-based approach where airlines must cap their emissions, or buy "carbon credits" from other industries.

The industry was overwhelmingly against the EU idea, said Tom Ballantyne, an analyst with Orient Aviation magazine, but was not necessarily opposed to being included in any post-Kyoto agreement.

"They realise that they are going to have to confront this," he said, adding the aviation industry had yet to decide on a united stance on climate change.

British tycoon Richard Branson, president of the Virgin Atlantic airline, recently flew from London to Amsterdam on a Boeing 747 partially fueled by coconut and a variety of palm oil.

He later extolled the energy potential of algae, press reports said, and has pledged millions of dollars in research into new technologies.

Many green activists are, however, lukewarm on the idea of expanded use of biofuels to curb transport emissions, saying that it could also cause new problems including pressure on food prices.

"There is no golden solution to the replacement of fossil fuels. It's impossible to replace one thing by a single other source," said Marcelo Furtado of Greenpeace Brazil.

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