Chinese President Hu Jintao offered few details in his UN speech on climate change, but the lack of specifics could just mean he wants to keep some room for manoeuvre, observers said Wednesday.

On Tuesday, Hu told the UN General Assembly in New York that China would curb the growth of its carbon dioxide emissions per unit of GDP by a "notable margin" by 2020 from their 2005 level, but did not give a figure for the cuts.

Hu could have stepped up pressure on developed nations ahead of global talks in Copenhagen in December by offering specific numbers, but may have deliberately opted not to do so, experts said.

"If Hu had done that, it would be a big push to the US position as well as the international climate negotiations that are going very slowly at the moment," said Greenpeace China climate change campaign manager Yang Ailun, referring to US hopes for firm commitments on the issue.

"But the fact that Hu didn't announce the target might mean that China wants to leave a bit more space for negotiation later," she told AFP.

Hu's description of the curbs in terms of gross domestic product reflects Beijing's paramount concern about the need to maintain rapid economic growth.

China and other developing countries have long resisted mandatory emission curbs becoming part of the next treaty on fighting global warming, saying rich countries bear historical responsibility for cleaning up the environment.

Developed nations, while pledging to fight global warming, have insisted that emerging powers also commit to action under an accord to be hammered out in Copenhagen as a successor to the Kyoto Protocol, which expires in 2012.

IHS Global Insight analyst Ren Xianfang described Hu's speech as a "symbolic gesture" to show China was serious about reducing emissions.

But the statement also showed Beijing was not prepared to commit to a target before the Denmark talks, she added.

"They are obliged to say something. The world needs China's commitment to this," Ren said.

"There's still a lot of uncertainty ahead of the big meeting in Copenhagen. China may be waiting for others to move first."

China, which is level pegging with the United States as the world's largest greenhouse gas emitter, has committed in its current five-year plan to cutting energy consumption per unit of GDP by 20 percent between 2006 and 2010.

The Asian giant, which relies on coal for 70 percent of its energy needs, also aims to get 10 percent of its power from renewable sources by 2010 and 15 percent by 2020.

Oxfam Hong Kong campaign coordinator Stanley So offered praise for Hu, saying he had shown "leadership to the developed countries" and "willingness" to take on responsibility for reducing emissions.

Greenpeace's Yang said Hu's speech was a positive sign that China was serious about tackling climate change.

"This was the first time that the Chinese government has publicly confirmed that China would have a carbon emissions reduction target. Before, it was just speculation," Yang said.

"So in this sense Hu's speech was still quite significant."