Jonathan Pershing also urged China, which has overtaken the United States as the top emitter, to clarify its goals for curbing its greenhouse gases as part of a new UN pact due to be agreed in December in Copenhagen.
Leading industrialized countries agreed at a summit in Italy in July that the world must halve greenhouse gas emissions by 2050 to avoid the worst effects of climate change, and promised to cut their own emissions by 80 percent.
China should cut by about 50 percent, which would allow for somewhat lower targets for poorer countries and give them room to grow their economies, Pershing told Reuters on the sidelines of the 175-nation talks in Barcelona.
"If you put China in there at a 50 percent reduction, if we're a bit higher, that gives lesser developed countries a bit lower. If they are in that middle band, plus or minus some percentage, that seems about right."
China would be on course to meet that goal if it repeated its present energy efficiency five-year plan into the future, he added.
"They're doing pretty well," he said. Beijing has not set a 2050 goal for its emissions, saying that it needs to put priority on ending poverty. The United States has not made a formal demand of China.
Pershing also said that the United States has not ruled out use of border tariffs if Washington feels that foreign exporters are getting an unfair advantage under a deal in Copenhagen under which carbon curbs push up U.S. energy costs.
"We cannot rule them out," he said. "The answer is that there is no decision."
The United States was "not opposed" to a legally binding climate treaty, under the UN process, Pershing added.