BEIJING – China unveiled on Thursday what it called an ambitious plan to boost energy efficiency and curb its carbon footprint in the most detailed indication yet of its stance heading into a world climate summit.

The world's most populous country will cut the amount of carbon dioxide emitted per unit of gross domestic product in 2020 by between 40 and 45 percent, based on 2005 levels, a statement from the State Council, or cabinet, said.

"This is a voluntary action taken by the Chinese government based on its own national conditions and is a major contribution to the global effort in tackling climate change," the statement said.

It added that China faced "enormous pressure and special difficulty in controlling greenhouse gas emissions".

The announcement marks the first time China has put specific numbers on a September pledge by President Hu Jintao to cut carbon intensity by a "notable margin".

Carbon intensity refers to emissions per unit of economic activity. Emissions would continue to grow under China's plan as economic growth is expected to continue. Beijing is not offering an absolute cut in carbon dioxide production.

The announcement came a day after Washington said President Barack Obama would attend the December 7-18 meeting in Copenhagen with an offer to cut US emissions by 17 percent from 2005 levels by 2020.

China said it will achieve its targets through a mix of measures including stepped-up efforts to develop renewable energy and "actively pushing forward nuclear power and other action".

It also said it would make "relevant" adjustments to "fiscal, tax, and pricing" policies, but gave no specifics.

Experts have called on China to raise the cost of energy, particularly heavily polluting coal on which it relies for about 70 percent of its fast-growing energy needs, to encourage conservation and curb emissions.

The government statement said China also would step up reforestation and investment in low-carbon and energy-saving technologies including "clean coal, renewable energy, advanced nuclear energy and carbon capture," it said.

Environmental group Greenpeace released a statement welcoming the announcement but called on both China and the United States -- the number one and two sources of carbon emissions -- to do more.

"Given the urgency and magnitude of the climate change crisis, China needs stronger measures," Greenpeace China's climate and energy campaign manager Yang Ailun said.

At a press conference after the plan was announced, China's top climate change negotiator Xie Zhenhua made clear his country's "primary task" remained economic growth, and that emissions would rise.

"China is a developing country, so the primary task we face is economic development, poverty alleviation and the improvement of people's livelihoods," he said.

He declined to offer a timeframe for when China's greenhouse gas output would peak or how big its emissions would be in 2020.

Greenpeace offered a harsh assessment of developed countries such as the US, which are viewed as having the wherewithal for more aggressive action.

Yang said the "inadequate" US offer amounts to just a 4-5 percent reduction from 1990 levels, compared to Europe's pledge to reduce emissions 20 percent from 1990 levels by 2020 -- and even more if a global deal is reached.

She said the developed world must cut emissions at least 25 to 40 percent below 1990 levels by 2020 and 80 to 95 percent by 2050 if the planet is to avert "catastrophic or runaway climate change."

Scientists have warned that without aggressive action, global temperatures could rise dramatically this century with calamitous consequences such as volatile weather and rising sea levels.

Pressure has mounted on China and the United States to announce substantive steps to limit emissions and seek a meaningful deal in Copenhagen.

China earlier Thursday announced that Premier Wen Jiabao would join dozens of other world leaders at the summit, which was called to seek a post-2012 successor to the Kyoto Protocol on limiting greenhouse gas emissions.

The United States wants aggressive Chinese steps to rein in its fast-growing emissions while Beijing argues rich developed nations bear the historical blame for carbon build-up in the atmosphere and should shoulder the burden.

China says that as a developing nation, it should be given leeway on emissions as it seeks to grow its economy and reduce poverty among its 1.3 billion people.