It came a day after the Group of Eight major industrial democracies set a goal of halving heat-trapping emissions that contribute to global warming by 2050.
The U.S.-led, 17-member group issued a final statement on the sidelines of the Group of Eight summit in northern Japan.
"We support a shared vision for long-term cooperative action, including a long-term global goal for emission reductions, that assures growth, prosperity, and other aspects of sustainable development," the expanded group said.
But the developing nations invited to the gathering were not ready to go as far as supporting the 50 percent reduction by 2050.
Jim Connaughton, chairman of President Bush's Council of Environmental Quality, said that "several" of the emerging economies were willing to support the target, but not enough to allow that language to be put in the declaration. He did not say which nations.
The White House did not speak of a setback; the fact that the group met as one and vowed to work together to reduce emissions "will give us greater confidence and commitments as we go to next year," said Connaughton.
The expanded group included China and India. They were invited to sit at the table with the Group of Eight: the U.S., Japan, Britain, Germany, France, Canada, Italy and Russia.
The statement on Wednesday also pledged to support a U.N.-led effort to conclude a new global warming pact by the end of next year.
Environmentalists, however, deplored the statement as meaningless without any targets.
"This whole initiative has been a wild goose chase and hasn't brought anything constructive to the U.N. talks," said Antonio Hill, of the aid group Oxfam International, an advocacy group that works on climate change and other causes.
Developing nations such as China and India have criticized the G-8's position statement for failing to state clearly what wealthy nations' commitments are, and that opposition was reflected in the lack of a longterm target in Wednesday's communique among the broader group.
Bush has pushed the so-called Major Economies Meeting to gather the countries most responsible for the greenhouse gases being emitted today.
Critics have attacked the grouping for excluding nations, such as small-island states, who will suffer most from the effects of global warming, such as rising sea levels.
In its own statement, the G-8 did not specify a base year for its proposed 50 percent cut, and the actual emissions reductions and the effect on the environment could vary hugely depending on what is eventually decided. Reductions from 2005 levels, for instance, would be far less than from 1990 levels, as in the Kyoto Protocol on global warming.
Still, U.N. Secretary-General Ban Ki-moon said it was essential to set a long-term goal for global greenhouse emissions by 2050. He said the world cannot afford to wait until 2009, when nations are planning to try to conclude a new global warming treaty to succeed the Kyoto Protocol when its first phase expires in 2012.
The United States has never ratified the Kyoto treaty, with Bush complaining that it puts too much of a burden on the U.S. and other developed countries to reduce emissions while developing giants such as China and India are given a freer rein to pollute even as they vigorously compete with America around the world.
Bush will leave office next January, and both major candidates to succeed him have said they are willing to go further in cutting back American emissions.
The G-8 statement solidified a pledge made at the last summit in Germany a year ago to seriously consider such a long-term target.
But the move fell far short of demands by some developing countries and environmentalists pushing for deeper cuts by 2050 and a firm signal from wealthy countries on what they are willing to do on the much tougher midterm goal of cutting emissions by 2020.
"To be meaningful and credible, a long-term goal must have a base year, it must be underpinned by ambitious midterm targets and actions," said Marthinus van Schalkwyk, South African Minister of Environmental Affairs and Tourism. "As it is expressed in the G-8 statement, the long-term goal is an empty slogan."