US president-elect Barack Obama on Tuesday vowed he would "engage vigorously" in global climate change talks and that denial was no longer an acceptable response to global warming.

Obama said in a surprise video message to an international conference on climate change hosted by five US state governors here that he would show new leadership on the issue as soon as he takes office in January.

The president-elect also addressed his message directly to delegates at United Nations climate change talks in Poland next month.

"While I won't be president at the time of your meeting and while the United States has only one president at a time, I've asked members of Congress who are attending the conference as observers to report back to me on what they learn there," Obama said.

"And once I take office, you can be sure that the United States will once again engage vigorously in these negotiations, and help lead the world toward a new era of global cooperation on climate change.

"Now is the time to confront this challenge once and for all. Delay is no longer an option. Denial is no longer an acceptable response. The stakes are too high. The consequences, too serious."

Obama said his administration would establish annual targets that would aim to reduce emission levels to 1990 levels by 2020, an ambitious goal that has already been adopted by California.

Obama's comments came at the opening of a two-day conference hosted by California Governor Arnold Schwarzenegger to discuss and develop strategies aimed at combating climate change.

Republican Schwarzenegger, who has repeatedly clashed with the outgoing administration of President George W. Bush over climate change and environmental issues, welcomed Obama's remarks.

"I'm very, very happy," Schwarzenegger told delegates, many of whom had cheered during Obama's taped speech.

"It's very important for our country, because we have been the biggest polluters in the world, it's about time that we as a country recognize that and work together with other nations to fight global warming."

Schwarzenegger also praised Obama's pledge to follow California's lead on cutting emissions levels. "This administration will adopt our regulations, and our laws. This is fantastic because we want to lead the way," he said.

Schwarzenegger is among five US governors co-hosting the event, along with the governors of Illinois, Kansas, Wisconsin and Florida.

The conference also featured some 800 officials from Australia, Brazil, Canada, China, the European Union, India, Indonesia, Mexico, New Zealand, Poland, Spain, United Kingdom.

Schwarzenegger has made the environment a key issue of his second term in California and signed a historic bill in 2006 that saw California become the first in the nation to impose limits on global warming gases.

Under the plan, California will aim to slash the state's carbon dioxide emissions by 25 percent by the year 2020, a figure that Schwarzenegger has said is equivalent to removing 6.5 million vehicles from the road.

On Monday, Schwarzenegger set a new target for California of securing 33 percent of its electricity from renewable sources by 2020.

The state had already set a goal of acquiring 20 percent of its electricity from sources such as wind and solar by 2010.