The country's top climate change official has rejected a proposal by the United States to slap tariffs on imports from countries that do not impose costs on carbon emissions.

The idea is an excuse for trade protectionism even as China boosts efforts to tackle greenhouse gases, Xie Zhenhua, head of China's climate change and coordinating committee, said on Wednesday in Washington.

"Climate change and imposing carbon taxes on imports ... are two separate issues" and should be tackled in separate negotiating forums, he said.