The Obama administration offered to curb US emissions by 17 percent from 2005 levels by 2020 - less than calls by the European Union, Japan and UN scientists but the first numbers on the table by the world's largest economy.

"The president going to Copenhagen will give positive momentum to the negotiations and we think will enhance the prospects for success," Carol Browner, Mr Obama's top aide on climate policy, told reporters.

Obama will address the meeting in Copenhagen on December 9, the day before he heads to Oslo to receive the Nobel Peace Prize.

The European Union has vowed to reduce its emissions by 20 percent from 1990 levels before 2020, raising the target to 30 percent in the event of an international agreement. Japan has offered 25 percent, but attached conditions.

AFP