The United States is the biggest source of greenhouse gases, mainly from burning fossil fuels, and Bush's decision to reject caps under the UN's Kyoto Protocol discourages involvement by other big-polluting outsiders such as China and India.

After two weeks of talks, about 70 environment ministers in Nairobi agreed on Friday to a 2008 review of Kyoto as a possible prelude to deeper emission cuts by rich nations beyond 2012 and steps by developing countries to brake rising emissions.

They also agreed modest schemes to help Africa adapt to the feared effects of climate change such as drought, storms, disease and rising seas. Ministers agreed to promote green technologies, such as wind or solar power, in the poorest continent.

Many said work on extending Kyoto was too sluggish when investors, for instance a firm building a coal-fired power plant, needed years to plan. Kyoto obliges 35 rich nations to cut emissions to 5 percent below 1990 levels by 2008-12.

"It is increasingly clear that global emissions will have to be halved by mid-century if we are to have a chance of keeping climate change within tolerable limits," said European Commissioner Stavros Dimas.

He said countries had to "step up efforts to complete the process as soon as possible".

Kyoto nations account for only 30 percent of emissions, mainly from factories, power plants and vehicles, and want outsiders led by the United States to take on cuts. The United States alone accounts for almost 25 percent.

"VERY COMMITTED"

Bush pulled out of Kyoto in 2001, saying caps would cost jobs and the plan wrongly omitted targets for poor countries.

"The president is very committed to the policy and strategy which he has set forth," said Paula Dobriansky, Under Secretary of State for Democracy and Global Affairs, who led the US delegation at the Nairobi talks, saying no shift was planned.

The next president will not necessarily change US policy but there is growing pressure from US states and cities for federal limits on greenhouse gases.

Three Democratic senators wrote to Bush on Wednesday saying they would push for "mandatory caps" on emissions. The three are to lead environment committees in Congress after winning control from Bush's Republicans in elections this month.

Several senior delegates at the UN talks say 2010 now looks the most likely date for a new global pact to replace Kyoto. "We'd love a deadline of 2008 or 2009 but that looks unlikely unless Bush has a change of heart," one said.

Environmentalists want a 2008 deadline. "Technically it's still not impossible," said Hans Verolme, climate director of the WWF conservation group. "The planet cannot wait."

Developing countries say rich states must lead the way -- China told Kyoto nations to makes pledges to cut their emissions in 2008 or 2009.

Reluctance to curb greenhouse gases also points to a weakness in a report last month by ex-World Bank chief economist Nicholas Stern, who said inaction over climate change might have apocalyptic economic costs compared with costs of acting now.

However, no countries want to go it alone, fearing they will have to pay now and will also suffer far bigger economic effects later if others fail to follow suit.