Russia offered to tighten curbs from 1990 levels to between 22 and 25 percent by 2020, according to Vladimir Chizhov, Russia's ambassador to the European Union, in some rare good news before a U.N. climate summit in Copenhagen next month.

Russia is the world's number three greenhouse gas emitter behind China and the United States. Its previous offer had been to aim for curbs of 10 to 15 percent below 1990 levels by 2020.

The offer, at a summit with the EU in Stockholm, would d eepen overall cuts by developed nations to between 13 and 17 percent below 1990 levels, from 11 to 15 percent before Moscow's announcement.

It was unclear how far Russia will seek to exploit the natural ability of its vast forests to soak up carbon dioxide as part of its new target.