TOKYO - Japan unveiled plans on Wednesday for greenhouse gas cuts over the coming decade only marginally deeper than its current U.N. commitments, a step green groups say threatens to deal a blow to global climate talks.

The world's fifth-biggest greenhouse gas emitter has been under pressure from developing nations to go for deep greenhouse gas reductions by 2020, to show leadership in talks on a new climate pact in December.

But Prime Minister Taro Aso, facing an election in a few months, had to balance fighting climate change with the needs of industry and the demands of voters during the nation's worst recession since World World Two.

The 2020 target is equivalent to a cut of 8 percent below 1990 emission levels. Under the Kyoto Protocol, Japan committed itself to cutting emissions by 6 percent from 1990 levels in the 2008-12 period, but has struggled to meet that goal.

It is an extremely ambitious target that aims to reach a 33 percent improvement in energy efficiency, more than the 30 percent rise in efficiency achieved during the oil crisis (in the 1970s), Aso told a news conference on Wednesday.

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U.N.-led climate talks are meant to result in a broader climate pact in December in Copenhagen to replace Kyoto after 2012. Hurdles include finding the money to finance a switch from fossil fuels and agreeing tough emissions caps for rich nations.

U.N. climate chief Yvo de Boer, steering the latest round of talks at a June 1-12 meeting in Bonn, said he was lost for words when asked to comment on Japan's target on Wednesday.

For the first time in two and a half years in this job I don't know what to say, he told reporters. Rich countries' collective carbon-capping plans for 2020 were so far nowhere near the targets recommended by climate science, he said.

Environmental groups were unimpressed, saying Tokyo had done little to make heavy industry rein in its planet-warming emissions from burning fossil fuels, and had set a bad example.

There's no effort to transform the economy, said Jennifer Morgan from the environmental group E3G. That's not consistent with where the science is on this issue. They're not inspiring China and other developing countries to go further. It's a race to the bottom.

Developing-country climate negotiators meeting in Bonn said Tokyo was ignoring rich countries' historical responsibility for contributing to climate change.

The main Japanese opposition Democratic Party, which leads opinion polls ahead of an election that must be held by October, has said emissions cuts of 25 percent from 1990 levels would be desirable.

Is this really a policy that will combat global warming? Democratic Party leader Yukio Hatoyama asked after Aso's announcement. I want Japan, as a leading technological power, to show more leadership, he added.

But some experts said the plan was reasonable.

It's balanced in light of domestic and global considerations, and in terms of Japan's initial stance going into future negotiations, said Satoshi Hashimoto, an analyst at Mitsubishi Research Institute.

Japan has struggled to meet its Kyoto carbon cap because offices and households increased emissions and power companies failed to meet their pollution reduction goals.

Aso said the 2020 target was tougher than targets set by other rich nations. The new goal excluded payments to fund emissions cuts abroad, for example, which are seen as a cheap option and are included under a corresponding European cap.

Tokyo's target was based solely on domestic cuts, from boosting energy efficient household appliances and cars and getting more energy from renewable sources such as the sun.

The European Union has promised to cut emissions by at least 20 percent by 2020 from 1990 levels, equivalent to a 14 percent cut from 2005 levels.

We encourage Japan to take further steps both domestically and internationally, said European Environment Commissioner Stavros Dimas.

In the United States, a draft climate bill approved by a congressional panel aims to cut emissions by 17 percent from 2005 levels.

(Additional reporting by Yoko Kubota, Mayumi Negishi and Isabel Reynolds in Tokyo, Gerard Wynn in Bonn, Chris Buckley in Beijing, Krittivas Mukherjee in New Delhi, Pete Harrison in Brussels; editing by Tim Pearce)