U.S. chief negotiator Todd Stern urged China to "stand behind'' its promise to slow the growth of the country's carbon output and make the declaration part of an international climate change agreement.
China rejected that demand, and renewed its criticism of the U.S. for failing to meet its 17-year-old commitment to provide financial aid to developing countries and to reduce emissions of carbon dioxide and other gases warming the Earth.
"What they should do is some deep soul-searching,'' said Yu Qingtai, China's chief climate negotiator.
The remarks during separate news conferences reflected the heavy lifting that remains in the 10 days before 110 heads of state and government conclude the summit, which aims to create a political framework for a treaty next year to succeed the 1997 Kyoto Protocol.
President Barack Obama helped break the ice in the troubled negotiations last month, saying he would deliver a pledge at Copenhagen to reduce U.S. greenhouse gas emissions by around 17 percent from 2005 levels by 2020. It will be the first time the U.S. has committed to a reduction target.
China responded a day later, announcing it would voluntarily reduce the carbon intensity of its industry by up to 45 percent, meaning its emissions would continue to grow but at a rate lower than the economy.
Stern said China's announcement boosted optimism before the conference, but didn't go far enough.
"What's important is not just that they announce them domestically but they put them as part of an international agreement,'' Stern said.
Whatever actions the Chinese take to slow emissions growth should be transparent, he said, "it's not just a matter of trust.''