The United States, the world’s largest producer of such gases, has resisted calls for strict limits on emissions at the UN climate conference, which is 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 percent 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 Bali.

"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 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.

The two-week conference, which opened Monday, is already in a tense standoff between two camps, with the majority supporting mandatory emissions cuts on one side, and opponents such as the United States on the other, delegates said.

Scientists say the world must act quickly to slash greenhouse gas emissions and limit the rise in global temperatures or risk triggering devastating droughts and flooding, strangling world food production and killing off animal species.

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 clamoring for dramatic action to stop global warming. U.N. climate chief Yvo de Boer led off his daily briefing Thursday by hailing the "encouraging sign" from the United States.

"This is a very welcome development," Alden Meyer of the Union of Concerned Scientists said of the Senate measure. "It shows the increasing isolation of the Bush administration in terms of U.S. policy on this issue."

David Waskow, of the Oxfam humanitarian agency, said the Senate legislation was a positive signal to developing nations and others in Bali that America may be ready to assume a more active role in battling climate change.

"It's one of the things that point the way to having the United States re-engage in the negotiations, and really I think in many ways demonstrates the U.S. leadership on these issues," Waskow said.

Further momentum for serious greenhouse gas cuts, 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 favor voluntary emission targets, arguing that mandatory cuts would threaten economic growth which generates money needed to fund technology to effectively fight global warming.

Indonesian Environment Minister Rachmat Witoelar, the host of the conference, said the mood in the closed-door negotiations was "serious, apprehensive," but that there were hopes the U.S. would slowly change its stance.

"I think the United States will be judicious enough to accept the changes of atmosphere," said Witoelar.

But U.S. Undersecretary of State Nicholas Burns denied that Australia's acceptance of the Kyoto accord would prompt Washington to do the same.

"We do not see eye-to-eye with Australia or many other countries on the wisdom of signing the Kyoto regime, that's obvious," Burns said in Sydney, Australia.