CANCUN, Mexico - The United States and China appeared close to agreement Wednesday on a key issue that has troubled climate change negotiations, boosting prospects that talks on global warming will score their first success in years.

Analysts said the tone over measuring emissions had softened between the two major protagonists in the 193-nation talks.

The much disputed issue involves how countries account for their actions to limit emissions of carbon dioxide and other heat-trapping gases and to what extent they will allow other countries to review their books. The process is known as measuring, reporting and verifying, or MRV in negotiating parlance.

Details remain to be worked out and MRV is only one of several elements that negotiators want to adopt as a package in Cancun.

"Maybe the differences are not that huge," said Su Wei, China's chief negotiator. "In general, both countries would like to promote the process" and emerge from Cancun with a deal.

The veteran diplomat said China had put in place a rigorous system for measuring and assessing its carbon emissions, and had no objection if other countries examined its reports. "We have no problem with MRV," he said.

Earlier this week, US negotiator Jonathan Pershing said the US and China had "spent a lot of energy in the past month working on those issues where we disagree and trying to resolve them. My sense is we have made progress." He did not specify those issues.