STOCKHOLM - U.S. President Barack Obama should do much more to ensure Congress passes a greenhouse emissions bill, giving global climate talks a major boost, the head of the U.N. Climate Change Panel said on Thursday.

Rajendra Pachauri, whose panel shared the 2007 Nobel Peace Prize with former U.S. vice president Al Gore, said legislation clarifying U.S. emissions targets would make all the difference to a climate conference in Copenhagen in December.

I personally feel that he ought to be doing a lot more, Pachauri told reporters on the sidelines of a conference, when asked about Obama's commitment to combating climate change.

I think that President Obama really needs to assert himself to see that the U.S. passes legislation -- it will make all the difference to negotiations, he said, referring to the Copenhagen talks on Dec. 7-18.

Pressure is growing before the Copenhagen conference for officials from 190 nations to agree a U.N. climate pact replacing the Kyoto Protocol, which runs out in 2012.

The European Union has already agreed to cut its greenhouse gas emissions by 20 percent below 1990 levels by 2020, but the United States has yet to pass similar legislation on its emissions targets.

While the House of Representatives has approved a 2020 target to cut emissions by 17 percent from 2005 levels, Congress as a whole has not approved any legislation, and analysts doubt that Obama will sign a bill by December.

European Commission President Jose Manuel Barroso said earlier this month that he was very worried time was running out before the Copenhagen conference.

The EU has pressured the United States to do more to secure a deal, and senior officials from the bloc will meet Obama in Washington early next month to discuss climate change, among other issues.

(Reporting by Nick Vinocur; editing by David Stamp)