WASHINGTON, Nov. 1 (Xinhua) -- A U.S. Senate subcommittee passed a global warming bill on Thursday, taking the first step towards a legislation that would impose mandatory cuts in greenhouse gases.

　　By voting 4-3, the Senate's global warming subcommittee greenlighted the bill that calls for limits on carbon dioxide and other greenhouse gases emitted from power plants, refineries, factories and motor fuels.

　　The subcommittee chairman, independent Senator Joe Lieberman, said that the vote "potentially a landmark event," showing "the United States finally began a serious fight against the threat of unchecked global warming."

　　The bill is expected to be included in the Senate Environment and Public Works Committee's agenda in coming weeks. The committee chairwoman, Democratic Senator Barbara Boxer, is an advocate for greater gas emission cuts.

　　If approved, the bill will be sent to the full floor.

　　The bill is supported by many environmental groups, but some Republicans argue that the emission requirements may already be too stringent.

　　The United States, the world's largest emitter of greenhouse gases, has refused to ratify the 1997 Kyoto Protocol, which mandates cuts in the greenhouse gases blamed for global warming.

　　President George W. Bush has voiced on many occasions his strong opposition to mandatory caps on greenhouse gas emissions.

　　"Each nation must decide for itself the right mix of tools and technologies to achieve results that are measurable and environmentally effective," Bush said when he addressed a U.S.-sponsored international climate change conference late September.

　　He also argued that the battle against climate change should not harm economic growth and "green" technologies and other voluntary efforts should be attached more importance to tackle global warming.