Yvo de Boer, said that the US role "would be critical" in the discussions, and that delegates must come up with a road map that Washington embraces.
De Boer, who is general secretary of the UN Framework Convention on Climate Change, also acknowledged that anyone who expects the Bali meeting to lead to specific targets or long-term solutions "will leave disappointed".
Ahead of the Bali talks, the US, which along with Australia refused to ratify Kyoto, said it was eager to launch negotiations.
But the Bush administration remains cool toward the idea of mandatory emission cuts.
The US argues that the Kyoto Protocol's mandatory cuts in emissions would harm the economy and has called cast doubts on the veracity of global warming science.
The Kyoto agreement bound 36 industrial nations to reduce carbon dioxide and other heat-trapping gasses emitted by power plants and other sources.
Now though the US may find itself isolated after the new Australian government, led by Kevin Rudd, put signing the Kyoto pact at the top of its international agenda.
Rudd, who was swept to power in elections just over a week ago, has also appointed his country's first climate change minister and has said that he and his new minister will travel to Bali for the talks.
|The Bali meeting is the world's largest ever |
conference on climate change [AFP]
The Bali conference follows a series of reports this year from a panel of international scientists which concluded that the world has the capability to slow global warming, but must act immediately to avert disaster.
The reports said the level of greenhouse gas emissions must be stabilised by 2015 and then decline to avoid the worst effects of climate change.
"The negotiations have to move faster and more decisively here in Bali," Kenya's environment minister, Kivutha Kibwana, the outgoing president of the conference, said before the start of talks.
"We have an opportunity… to save the environment and save the planet Earth for future generations."
Aside from the question of US involvement, the Bali meeting will also have to address the extent to which up-and-coming economies such as China and India will have to rein in their skyrocketing emissions.
Also high on the agenda will be measures to help the world's poorest countries adapt to a worsening climate.
The UN says a new agreement to replace Kyoto must be reached within two years to give governments time to ratify it and to ensure a smooth, uninterrupted transition.