Climate change campaigner and former vice president candidate Al Gore today accused the US of blocking progress at the Bali talks.
But he remained optimistic that a breakthrough was possible in the final days of the conference aimed at finding a successor to the Kyoto agreement.
"Some of the reports are worrisome, but I know from experience ... that when breakthroughs do occur, they usually happen in the last 48 hours," the Nobel Prize winner said.
UN Secretary-General Ban Ki-moon, addressing the meeting in Bali last night
"I hope there will be a change on the part of some countries, including most importantly my own, the United States."
UN Secretary-General Ban Ki-moon earlier urged world leaders to agree a sweeping treaty to fight climate change by 2009, telling the UN-led talks to act now on "the moral challenge of our generation".
The European Union and developing nations strongly favour specific target ranges for emissions, but the US has argued strenuously at Bali that including such language in the final document would prejudice negotiations over the next two years.
Some developing nations, worried that any commitments to curb fossil fuel use might slow economic growth, want Bali to launch only non-binding talks. And the US opposes many other nations' hopes for the guiding terms for negotiations to include a non-binding range for rich countries to cut greenhouse gases by between 25 and 40 per cent below 1990 levels by 2020.
"The position of the administration in the US right now appears to be to try to block any progress in Bali. I hope that will change," Mr Gore said.
Mr Gore and the UN's chief climate scientist Rajendra Pachauri will travel to Bali today for the ending of the two-week climate change conference on Friday.
Meanwhile in Bali UN Secretary General Ban Ki-moon said a call for rich nations to slash greenhouse gas emissions by 2020 may be "too ambitious" to include in the final agreed international statement on climate change.
Drafts of the statement have included guidelines for industrialised countries to consider cutting emissions blamed for rising temperature by between 25 per cent and 40 per cent.
But Mr Ban said those levels might have to wait for subsequent negotiations, though at some point targets for emissions cuts would be necessary.
Talks picked up pace today with the arrival of ministers and heads of state who urged the world in a series of speeches to quickly impose deep cuts in emissions to head off scientific predictions of rising seas, worsening droughts and famines, and melting ice-sheets.
The EU and developing nations strongly favour specific target ranges for rich countries' emissions, which supporters say are needed to avoid temperatures rising above 2C over pre-industrial levels.
The United States, however, has argued strenuously that including such language in the final document now would prejudice negotiations over the next two years.
Many negotiators at Bali have said that a top priority was getting an agreement that the United States could work with.
Delegates laid out their vision of an agreement: that wealthy nations, as the prime drivers of global warming, should make the first cuts in emissions and help poorer countries develop in a clean way with technology and assistance. They also called on rapidly developing nations, such as China, to rein in high levels of pollution.
Yesterday, Australia's Prime Minister Kevin Rudd handed formal papers to Mr Ban ratifying the Kyoto Protocol, isolating the United States as the only rich nation without binding caps on greenhouse gas emissions under the UN deal stretching to 2012.
The Kyoto Protocol commits 36 industrialised nations to reduce greenhouse emissions by an average of 5 per cent between 2008 and 2012, but the pact has been severely weakened by Washington's refusal to join.