American climate change official Todd Stern has stepped off the plan in Copenhagen and into an immediate row with the Chinese delegation.

Stern has responded to China's delegation deputy chief Su Wei's criticism of the preliminary proposals made by the US for greenhouse gas emissions reductions.

Su Wei has said that the present target announced by the US is only one percent below 1990 levels.

"I'm not very good at English, but I doubt whether just a one percent reduction can be described as remarkable or notable," Su was quoted as saying by The Australian.

But Stern hit back at his first press briefing at the 192-nation UN Climate Change Conference.

"Right now China is the biggest emitter in the world," he said, adding that its emissions were going up "dramatically, really dramatically."

He claimed his comments were not a critical of China but "basic maths."

"We can't reach an agreement if China won't make major cuts. That's just the reality," Stern said.