VIENNA - Nations aiming to agree on a new global climate deal should focus on achievable greenhouse gas emissions targets, to involve as many nations as possible, said U.S. Energy Secretary Steven Chu.

The world is meant to thrash out in December in Copenhagen a new international climate change pact beyond 2012, to replace the Kyoto Protocol.

Developing nations want rich countries to cut emissions by 25 to 40 percent from 1990 levels by 2020 to avert the worst effects of climate change. But many industrialized nations fear such cuts are out of reach, especially in an economic downturn.

Chu said the outcome of the Copenhagen talks must not be so weak that they are a waste of time but added they should not be seen as the last chance to act on climate change.

Do I say let us wait for it to be overwhelming? No. You have to bring more people along, so don't tee it up as now or never, he told reporters.

What the United States can bring and can agree to is certainly unknown but I think probably 40 or 30 percent (cuts) might be too aggressive for 2020 for the United States.

He said at a briefing in Vienna that the goal would be too ambitious to get through the U.S. Senate. Chu said it was important to set realistic climate targets.

If we look at the history of Kyoto where many countries did sign on and many of the countries have not met their goals, one has to look at how to really get there, Chu said.

He said that if developed countries set achievable targets and improve efficiency, they can set an example and dispel fears that green policies can hamper the economy.

If you could get all those gains in the first 20, 30 percent reduction in carbon (emissions), just by using energy efficiently, you can teach people that there is a path, he said.

The United States, the world's second biggest carbon emitter, has proposed cutting its emissions to 1990 levels by 2020 -- a reduction of 14 percent from 2007 levels.

Chu acknowledged that unless countries agreed on ambitious emissions targets soon, they would have to become far more aggressive later to combat the worst effects of climate change.

Do I think the developed world can decrease their carbon emissions by 20, 30, 40 percent? Yes. I think it is a very, very aggressive goal, it is achievable, Chu said. But he added that targets need to be sold at home and to the rest of the world.

Russian Prime Minister Vladimir Putin has said he would reject any new climate change pact that imposed restrictions on Russia but not bind other big polluters, like the United States or China.

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