Japan's announcement on Monday of a 25-percent 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 (IIED) in London, said Japan's move would hike pressure on other major players ahead of the United Nations' talks on 7-18 December.

"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-percent 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 (UNFCCC), aim to craft a post-2012 pact for curbing the heat-trapping gases that drive perilous global warming.

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