Annan had urged rich countries at the 189-nation talks in Nairobi to be more "courageous" in cutting greenhouse gases and urged Washington to reconsider opposition to the UN's Kyoto Protocol that binds 35 nations to cut emissions by 2012.

Dobriansky said the United States was sticking to policies focused on braking -- rather than cutting -- the rise of emissions while investing heavily in green technologies such as hydrogen or new technologies for cleaner burning of coal.

"We seek to slow, reverse and really curb emissions," she said. The United States is the world's number one source of heat-trapping emissions from burning fossil fuels in power plants, factories and cars ahead of China, Russia and India.

Annan said Kyoto was a small first step to fight global warming. "And as we consider how to go further still, there remains a frightening lack of leadership". He said the criticism was not aimed at any single nation.

Big developing nations, such as China and India, had to start braking their own surging emissions from power plants, factories and cars, Annan said. Kyoto excludes developing nations from targets for 2012.

The Nairobi talks are seeking ways to extend Kyoto beyond 2012 and to step up aid to poor nations, especially Africa.

AID FOR AFRICA

Annan unveiled a plan by six UN agencies, dubbed the "Nairobi Framework", to help poor nations, especially in Africa, get more funds for clean energies such as wind and hydropower.

UN development and environment agencies would also advise poor nations about how to "climate proof" crops or infrastructure, for instance by placing coastal roads further inland to avoid being swamped by rising seas.

The Ghanaian urged rich donor nations to contribute.

Environmentalists say even backers of Kyoto -- such as the European Union -- are showing too little urgency in finding a successor to it. A document on Wednesday said the ministers would set no deadlines for agreeing how to extend Kyoto.

"It was the minimum needed to keep this process moving forwards," said Jennifer Morgan of the British-based environmental think-tank E3G. Many environmentalists want a deal on how to extend Kyoto by 2008 to give investors time to adapt.

German Environment Minister Sigmar Gabriel said that a previous agreement on Tuesday about a Kyoto fund to help poor nations adapt to climate change -- now totalling US$3 million -- was too skimpy for a meeting that has cost US$4 million to host.

"Do we really need to assemble 6,000 participants to decide on the structure of a relatively small fund?" Gabriel asked in a speech. "Should we not finally start giving adequate responses to one of the biggest challenges of mankind?"

The European Union said it was leading an assault to combat climate changes that could bring more floods, droughts, desertification and rising sea levels.

"We want to be front-runners in this debate," Gabriel said. Kyoto obliges 35 countries to cut emissions of heat-trapping gases to 5 percent below 1990 levels by 2008-12.

President George W. Bush said in 2001 that caps on emissions would harm the US economy and that developing nations were wrongly omitted from goals for 2012. But Annan said: "It is increasingly clear that it will cost far less to cut emissions now than to deal with the consequences later." (Additional reporting by Gerard Wynn)