The Kyoto protocol was adopted in Japan in 1997 by major emitting countries, committing them to cutting 1990 emission levels by an average 5 percent by 2012.
Japan's Wednesday evening announcement, made by an official in the government's economics trade and industry department, was the strongest yet against the protocol by one of the largest emitters of greenhouse gases, the British newspaper The Guardian reported.
"Japan will not inscribe its target under the Kyoto protocol on any conditions or under any circumstances," Jun Arima said.
The statement caught other delegations at the conference by surprise.
"For Japan to come out with a statement like that at the beginning of the talks is significant," a British delegate said. "The forthrightness of the statement took people by surprise."
If it is in fact Japan's new, formal position rather than a negotiating tactic, diplomats say, it could provoke a walk-out by some developing countries and threaten a breakdown in the talks.
"This is a very bad start to the negotiations. The danger is that other countries may want to follow Japan's example and run away from binding commitments to cut emissions," Poul Erik Laurisden, a spokesman with Care International, said.