Japan is likely to abandon an ambitious pledge to slash greenhouse gas emissions by a quarter, the top government spokesman said Thursday.

Asked to confirm if the new administration would review Tokyo's 2009 pledge, Chief Cabinet Secretary Yoshihide Suga said the government was "moving in that direction in principle".

"I have been saying for some time that it is a tremendous target and would be impossible to achieve," he told a regular news conference.

Then-prime minister Yukio Hatoyama made the pledge in 2009, following a landslide election victory by his centre-left Democratic Party of Japan.

It was lauded by environmentalists as one of the most ambitious of any industrialised country.

Hatoyama said the nation would slash its carbon emissions by 25 percent from 1990 levels by 2020, provided other major polluters such as China and the United States also made sharp reductions.

But officials say the pledge will be difficult to fulfil because of the huge rise in fossil fuel use since the nuclear disaster at Fukushima put Tokyo's atomic energy programme on hold.

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 elections after pledging to review the emissions cut target in light of the post-Fukushima switch to fossil fuels.