China hopes its emissions of greenhouse gases will peak "as early as possible" but its experts differ as to when it will happen, its chief negotiator at UN climate talks said Thursday.

Speaking to journalists at the latest round of negotiations, Xie Zhenhua, vice chairman of the National Development and Reform Commission, said China was working hard to boost its energy efficiency and expand the role of cleaner renewable sources.

"China is doing its utmost to reduce its carbon intensity," said Xie.

"However, you have to bear in mind that China is in the process of realising modernisation, and the total amount of CO2 will be increasing in the future."

A voracious burner of coal to provide the energy for its rise out of poverty, China is now the world's biggest single contributor of heat-trapping greenhouse gases.

The surge is a big reason why scientists fear that hopes of curbing global warming to a safer two degrees Celsius (3.6 degrees Fahrenheit) may be elusive.

"China will do its utmost to peak as early as possible," Xie said, speaking via an interpreter.

"If asked whether China is thinking about when will the emissions be peaking, yes we are studying about that and we are trying to find the date."

But, he said, "the peaking year is a very complex issue and related very closely to economic development, social development and environmental issues."

Chinese experts were still wrangling over the answer, said Xie.

"This process has been going on for more than one year and I can tell you that opinions of scientists and scholars differ quite a lot," he said.

"We are working very hard and try to find a balanced equilibrium and economic development and environmental protection."

"We hope that we can find an answer to that issue as early as possible," he added.

Xie said that China and the United States, the world's second-largest emitter, are working well together on climate change.

He noted that the two countries are in different stages of development and bore different responsibility for past global warming.

"But one thing in common is that we are both working very hard to address climate change," he said.