Japan's announcement of a 25 per cent cut in its greenhouse gas emissions could be a game-changer at the UN showdown on climate change in Copenhagen in December, observers said.
It could sweep away the who-jumps-first obsession that has bedevilled the world climate talks for nearly two years, they said.
"For a long time, everybody has been waiting for everybody else to move in the negotiations.... At this crucial point, the strong message from Japan is exactly what is needed," said Denmark's Climate and Energy Minister Connie Hedegaard.
"The decision by an important player such as Japan to do more and get serious about a low-carbon future can help break the deadlock between developed and developing countries," said Kim Carstensen of green group WWF.
Saleemul Huq, senior fellow at the International Institute for Environment and Development in London, said Japan's move would hike pressure on other major players ahead of the UN talks between December 7 and 18.
"It is a very significant step forward," he said.
"The logjam is beginning to be broken. The EU has now been joined by Japan. There's going to be a lot of behind-the-scenes words with other countries to take action."
Breaking dramatically with the policies of his conservative predecessors, Japan's incoming centre-left premier Yukio Hatoyama said his government would seek to cut the country's carbon emissions by a quarter by 2020 from 1990 levels.
Although important details remain sketchy, the 25 per cent target is the most ambitious mid-term target set so far by a large, advanced economy and the first to meet a threshold set by UN scientists.
The Copenhagen talks, under the 192-nation UN Framework Convention on Climate Change aim to craft a post-2012 pact for curbing the heat-trapping gases that drive perilous global warming.
But progress has stagnated amid foot-dragging and finger-pointing.
Broadly speaking, rich countries want China, India and Brazil - already major polluters and set to be the big emitters of tomorrow - to sign up to strong commitments for tackling their gases.
But the poorer countries say rich economies bear the historical responsibility for warming and should show goodwill by offering deep emissions cuts of their own. Until now, the lead has been taken by the European Union, which has uniterally decided to cut its emissions by 20 per cent by 2020, and offered to deepen this to 30 per cent if others follow suit.
Japan, under the outgoing government, had proposed a reduction of eight percent over 1990. The US would see a reduction of about four per cent, under a bill going through Congress.
Mr Huq said it was unclear whether Mr Hatoyama's announcement would trigger a deeper EU cut, as the mechanism by which the Europeans would extend their offer is unclear.
Greenpeace International's Martin Kaiser hoped Mr Hatoyama's "climate leadership" would be emulated by US President Barack Obama.
But he and others also sounded a note of caution, seeing apparent conditions attached to the announcement and fearing the blocking ability of Japan's powerful business lobby, which wants a cut of no more than six percent.
Mr Hatoyama said a "pre-requesite" for Japan was "a highly ambitious accord with participation by all major countries."
UNFCCC executive secretary Yvo de Boer praised Mr Hatoyama's target for being "commensurate with what science says is needed."