Japan said Friday it was dramatically scaling back its greenhouse gas reduction target after the Fukushima nuclear accident forced the country to turn to fossil-fuel burning energy sources, a move denounced by climate campaigners.

Tokyo said the new target for 2020 -- 3.8 percent below 2005 levels -- replaces an ambitious goal to slash emissions by one-quarter from 1990 levels.

The new target, which accounts for idling the country's nuclear reactors after the worst atomic accident in a generation, represents about a three percent rise over levels in 1990, the base year for the Kyoto Protocol, according to the environment ministry.

Chief Cabinet Secretary Yoshihide Suga, the government's top spokesman, said the earlier target set in 2009 by a centre-left government under then-prime minister Yukio Hatoyama was never realistic.

"Our government has been saying... that the 25 percent reduction target was totally unfounded and wasn't feasible," he told reporters in Tokyo.

The previous target had almost no road map for achieving the goal outside of increasing Japan's dependence on now-shuttered nuclear power, Tokyo said, calling its new blueprint "an aggressive diplomatic strategy on climate change".

Hatoyama had said the nation would slash its carbon emissions provided other major polluters such as China and the United States also made sharp reductions.

Japanese environment minister Nobuteru Ishihara is expected to announce the new target next week at a 12-day UN climate conference in Warsaw which kicked off on Monday.

"This is Japan's new international commitment,... which will be registered at the United Nations," a foreign ministry official said Friday.

He added that the target was "temporary" given Tokyo's desire to switch on idled nuclear reactors, an idea that has been met with strong public resistance.

The earthquake and tsunami of March 2011 sent reactors at the Fukushima nuclear plant into meltdown and generated widespread distrust of a technology previously relied on to provide around a third of Japan's electricity.

Prime Minister Shinzo Abe's business-friendly Liberal Democratic Party ousted the Democratic Party in December 2012 elections after pledging to review the emissions cut target in light of the post-Fukushima switch to fossil fuels.

The move has sent Japan's energy bills soaring and set down a new challenge for efforts to cut greenhouse-gas emissions.

New target could set off 'race to the bottom'

Conservation groups condemned the pullback on Japan's commitments, saying it could dent progress at the UN climate talks in Poland.

Tokyo's new target "is absolutely unacceptable as it makes light of its (2009) international pledge", Greenpeace said.

"It is possible to realise both an exit from nuclear power and a reduction in greenhouse gas emissions.

"The government has establish a clear target and roadmap to prompt the introduction of natural energy and spreading of energy-efficiency technology," it added.

Environmental group WWF said the move "could have a devastating impact on the tone of discussions... in Warsaw".

"It could further accelerate the race to the bottom among other developed countries when the world needs decisive and immediate actions to 'raise' ambition, not to 'lower' ambition."

Abe's administration has announced plans for big investments in so-called green energy, such as wind and solar power, to help make up the power gap left by its zero-nuclear status.

But such technology currently makes up a tiny part of Japan's energy mix.

Separately, the foreign ministry confirmed that Tokyo would pledge about one-third of the expected $35 billion in aid that developing nations were expected to ask for to battle climate change over the next few years.

The climate talks in Warsaw are aimed at setting a timetable and agreeing steps towards a global and binding agreement on climate change to be signed in Paris in 2015 and enter into force in 2020.

That pact would for the first time bind all the world's nations -- including China and the United States, the number one and number two nations in size of emissions -- to measurable targets for curbing greenhouse gas emissions blamed for global warming.