American climate negotiators refused to back down in their opposition to mandatory cuts in greenhouse gas emissions Thursday, even as a U.S. Senate panel endorsed sharp reductions in pollution blamed for global warming.

The United States, the world's largest producer of such gases, has resisted calls for strict limits on emissions at the United Nations climate conference in Bali aimed at launching negotiations for an agreement to follow the Kyoto Protocol when it expires in 2012.

That stance suffered a blow when the Senate environment and public works committee passed a bill Wednesday to cut U.S. emissions by 70 per cent by 2050 from electric power plants, manufacturing and transportation. The bill now goes to the full Senate.

U.S. climate negotiator Harlan Watson, however, said that would not impact Washington's position at the international gathering.

"In our process, a vote for movement of a bill out of committee does not ensure its ultimate passage," he told reporters. "I don't know the details, but we will not alter our posture here."

It was the first bill calling for a mandatory U.S. limit on greenhouse gases to be taken up in Congress since global warming emerged as an environmental issue more than two decades ago.

Republican critics of the bill argued that limiting the emissions could become a hardship because of higher energy costs.

Washington's isolation in Bali has increased following Australia's announcement Monday that it has reversed its opposition to the Kyoto pact and started the ratification process — winning applause at the conference's opening session.

That left the U.S. as the only industrialized nation to oppose the agreement.

The U.S. Senate action cheered environmentalists and others in Bali clamouring for dramatic action to stop global warming. UN climate chief Yvo de Boer led off his daily briefing Thursday by hailing the "encouraging sign" from the United States.

Further momentum for serious greenhouse gas cuts, which experts say are needed to stave off the most destructive effects of rising temperatures, came from a petition released Thursday by a group of at least 215 climate scientists who urged the world to reduce emissions by half by 2050.

"We have to start reducing greenhouse gas emissions as soon as we possibly can," said Australian climatologist Matthew England, a group spokesman. "It needs action. We're talking about now."

The United States and ally Japan are proposing that the post-Kyoto agreement favour voluntary emission targets, arguing that mandatory cuts would threaten economic growth that generates money needed to fund technology to effectively fight global warming.