TOKYO — The U.N. climate chief today praised President Barack Obama's pledge to tackle global warming and expressed hope that the U.S. policy shift would boost chances for a new international agreement on cutting emissions of greenhouse gases.

"It's been a night-to-day change in terms of the U.S. position on this topic," United Nations climate chief Yvo de Boer said in Tokyo, adding he hopes the more active American approach will encourage China and other developing nations to make further efforts to control their emissions.

De Boer was in Tokyo to attend two days of international talks on laying the groundwork for negotiations on a new global agreement on cutting carbon emissions in December in Copenhagen.

The U.S. position is seen as crucial for the outcome of the Copenhagen meeting.

Under former President George W. Bush, the United States refused to sign the last climate treaty, the 1997 Kyoto Protocol, citing a lack of participation by developing countries, particularly China, and harm to the U.S. economy.

As a result, the U.S. and China became at odds, with each side refusing to take action on global warming without the other taking a step, falling into "a chicken and egg situation," De Boer said.

"Now we see the United States coming back to the international negotiations," he said. "I believe that a Sino-American way forward on trade, on technology and on climate change could be a major contribution and impulse to the broader negotiating process."

By The Associated Press