The Obama administration is "fervently engaged" in UN talks to forge a global climate treaty but cannot rescue the troubled process on its own, the top US climate negotiator said on Sunday.
"Yes, the US will be powerfully and fervently engaged in this process," Todd Stern said as the 11-day United Nations Framework Convention on Climate Change (UNFCCC) forum got under way.
But in his first press conference here as Special Envoy on climate for US President Barack Obama, Stern cautioned that the US could not "wave a magic wand" to reconcile difference.
"I don't think anybody should be thinking that the US can ride in on a white horse and make it all work," he said.
Stern was enthusiastically applauded when he addressed the more than 2500 participants at the UN forum. "We are very glad to be back," he told the delegates.
His welcome could in part be explained by the fact that Obama's predecessor, George W. Bush, rejected the Kyoto Protocol, whose provisions expire in 2012. Bush also refused to commit the United States to fixed targets for cutting carbon pollution.
The entry of the new US team into a process involving more than 190 states and riven by deep divisions between rich and developing countries has generated huge expectations.
But there are less than nine months left on the existing timetable to complete an agreement which will be of extraordinary complexity.
Stern challenged China and other emerging economic powerhouses such as India and Brazil to take on stronger commitments in curbing carbon pollution.
China and the US between them account for more than 40 percent of the world's greenhouse gas emissions.
"If you do the math, you simply cannot be anywhere near where science tells us we need to be if you don't have China involved, as well also other major developing countries," he said.
"How that is captured, understood, expressed and quantified is going to be extremely important," he said.
The other key issue to be resolved, he added, was how to divide up the cost of mitigating greenhouse gases and adapting to the devastating impacts of global warming.
Industrialised nations are prepared to take on the larger burden, but want emerging economies that are also major carbon polluters to undertake action of some kind.
These countries, in turn, say rich nations should take the lead in making deep cuts, and put money on the table to help them develop clean technology and adapt to climate change already underway.
Stern rejected criticism that Obama's national targets were not ambitious enough, or that they fell far short of European efforts.
The EU has promising to slash emissions by 20 percent compared to 1900 levels by 2020, and by 30 percent if other industrialised countries follow suit. By 2050, the cuts would be deepen to 80 percent.
During the US presidential campaign, Obama vowed to match the European Union's mid-century objectives.