US carmakers have said they need help in tackling the challenges the industry is facing set by global warming.
The heads of General Motors, Chrysler and Ford said that measures to improve fuel efficiency were not enough and may cost thousands of jobs.
They told a House of Representatives subcommittee that the car industry alone could not act on climate change.
US car firms are facing major struggles as they try to reshape their business in the face of stiff competition.
"New vehicle efficiency improvements alone will never result in the overall decline in petroleum consumption and greenhouse gas emissions"
The car makers were joined by Toyota and the United Autos Workers (UAW) Union in the meeting with politicians at a time when global warming and ways of tackling it are high on the global agenda.
'Calamitous'
Meeting a target to boost fuel efficiency by 4% would be "extraordinarily expensive" and "technologically challenging" said GM's head Rick Wagoner.
He added that even if the target were met, the US would be using more oil and producing more carbon dioxide emissions than ever before.
And UAW President Ron Gettelfinger said that raising fuel economy standards "could lead to calamitous results "including tens of thousand of automotive jobs".
President and chief executive of DaimlerChrysler's Chrysler Group, Tom LaSorda, was also adamant that a multi-pronged approach was needed.
He called for more alternative fuels such as ethanol and bio diesel to be used.
"We all need to be very clear on one point - new vehicle efficiency improvements alone will never result in the overall decline in petroleum consumption and greenhouse gas emissions we need," Mr LaSorda said.
Toyota's North America president, Jim Press, said the industry had "a responsibility to be part of the solution" but that "these issues cannot be addressed by the industry alone".
Last month European car makers described plans to force them to cut back on harmful exhaust emissions as "unbalanced" and "damaging".
The industry body said EU targets were "arbitrary" and would lead to job cuts and relocation of production abroad.
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