The United States, butting heads with the Europeans, has opposed setting any definite commitments at the July 7-9 summit for slashing carbon output blamed for causing global warming.
Japanese Prime Minister Yasuo Fukuda conceded that the G8 -- Britain, Canada, France, Germany, Italy, Japan, Russia and the United States -- would not set targets for the period after the Kyoto Protocol's obligations end in 2012.
"Agreeing on a medium-term target is the core challenge for the UN negotiations that will take place up to the end of 2009," Fukuda told a group interview with news agencies from the G8 nations.
"The G8 is not a forum to agree on that target," he said.
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Japan has also invited leaders of other major greenhouse gas emitters, including China and India, for an expanded summit on the final day of the G8.
"What is important is that all major economies participate in a responsible manner. We would like to strive to come up with a political outcome that will encourage the realisation of an effective post-Kyoto framework," Fukuda said.
"We will strive to engage in constructive discussions so that we will be able to come up with a concrete outcome with regards to long-term targets," Fukuda said.
The United States, the only major industrial nation to reject the Kyoto Protocol, has said any solution on climate change must involve all countries, including developing states.