BONN, Germany (AP) - The chief U.S. climate negotiator says some countries are backing away from agreements they made at a summit meeting last December on a global warming deal.

Jonathan Pershing declines to name those countries, saying his view comes from closed-door meetings.

But after five days of negotiations in Bonn, Pershing says he is "very concerned" about the prospects for an agreement at a climate convention in Mexico at the end of the year.

Pershing told reporters Friday the U.S. remains committed to its pledge at the Copenhagen summit to reduce greenhouse gas emissions, despite the failure of the Senate to bring a comprehensive bill to the floor for debate.

He said President Barack Obama has not given up on passing legislation, but other tools are available.