Hillary Clinton, the US secretary of state, has said Washington "is ready to lead" the global fight against climate change, and no longer doubts the urgency or size of the problem.

"The United States is fully engaged and ready to lead and determined to make up for lost time both at home and abroad," Clinton told an international forum in Washington on Monday.

Clinton was believed to be alluding to the administration of George Bush, the previous US president, which faced widespread criticism for playing down the threat of climate change.

During Bush's tenure, the US refused to ratify the Kyoto agreement on climate change, branding it unfair since it made no demands on rapidly developing economies such as China and India.

"The United States is no longer absent without leave," Clinton said.

New agreement

The Major Economies Forum on Energy and Climate was attended by the countries responsible for 75 per cent of the global emissions of heat-trapping gases.

Its stated aim is to lay the groundwork for an international agreement to slow climate-changing pollution.

Delegates from 175 countries will meet in Copenhagen in December to forge a new treaty to replace the 1997 Kyoto Protocol, which expires in 2012.

The Kyoto Protocol, which required 37 countries to cut emissions by a total of five per cent by 2012, was not ratified by the US.

Clinton told the forum that the US was "working tirelessly" to ensure that an agreement could be reached on the issue this time.

But she also hinted that developing countries such as India and China would also have to be included.

"Of course, each economy represented here is different. And some, like mine, is responsible for past emissions, some for quickly growing present emissions," she said.

"But people everywhere have a legitimate aspiration for a higher standard of living. We just hope we can work together in a way to avoid the mistakes that we made that have created a large part of the problem that we face today."