Barack Obama's new climate change programme may be attacked by some activists as too modest, but it could still turn into a significant leap if it results in a worldwide agreement that includes China.
The plan, unveiled on Monday, would see a reduction in carbon dioxide emissions from US power plants - many of which are coal-fired - of 30 per cent from 2005 levels by 2030.
The initiative may be a crucial move in pressuring Beijing to accept binding goals to cut greenhouse gases, while also allowing the US to start catching up with the European Union in the fight against climate change.
A senior adviser to Beijing on climate change said on Tuesday that the notion of imposing an absolute cap from 2016 was "my personal view" and not yet official policy.
He Jiankun, chairman of China's Advisory Committee on Climate Change, had earlier told a conference in Beijing the government intended to impose a cap as part of policy to reduce emissions. But He explained later: "What I said today was my personal view. The opinions expressed at the workshop were only meant for academic studies. What I said does not represent the Chinese government or any organisation."
Nevertheless, the moves by Obama gave rise to optimism.
"This is the kind of leadership that's highly needed," said Martin Kaiser, climate politics chief for Greenpeace. The proposal should have been twice as ambitious, he added, but "it demonstrates that the Obama administration wants to seriously tackle climate change".
Governments want an agreement by late next year in Paris to curb emissions of greenhouse gases blamed for global warming.
Unlike the 1997 Kyoto Protocol, which exempted developing nations from emissions limits, this deal is supposed to cover every country.
The US never ratified the Kyoto Protocol, handing China and others an easy excuse to dodge tougher action as well.
"The new initiative is a first firm commitment that puts the US in a serious negotiating position for the upcoming climate talks in Paris," said Georg Zachmann, an expert with the Brussels-based think tank Bruegel.
"It gives hope that further steps in that direction will be forthcoming," he added.
The US announcement came ahead of international climate talks that started yesterday in Bonn, Germany, where governments were discussing their ambitions for tackling global warming over the coming decades.
Climate change is also on the agenda at a meeting of the G-7 leaders in Brussels today, bringing together Obama and his counterparts from the other leading Western economies.
China, the world's biggest polluter ahead of the US, has pledged to curb its output, but has so far resisted binding limits.
"Obama's plan to cut greenhouse gas may have some impact on China's decision-making," said Wang Ke, a professor at the School of Environment and Natural Resources at People's University in Beijing.
"But China's goal will be based on its domestic needs in the transformation of its economy and handling smog."
United Nations climate chief Christiana Figueres insisted she fully expected "action by the United States to spur others in taking concrete action".
Greenpeace's Kaiser sounded an optimistic note on China, saying that witnessing high pollution in their own cities had convinced leaders in Beijing that only joint action could tackle the problem.
Like many developing countries, China's status has changed drastically since the Kyoto Protocol. It has grown into an export powerhouse and the world's second-largest economy, prompting US lawmakers to say any new treaty must cover China.
Beijing has said it is still too poor to take on the limits imposed on rich countries.
China accounted for 29 per cent of global carbon emissions in 2012, more than the US and the European Union combined, according to the Netherlands Environmental Assessment Agency.
Obama has pledged a cut of 17 per cent of the entire US economy's emissions by 2020 compared with the level in 2005.
Additional reporting by Reuters