Such a move, Europeans say, could pressure Bush to relent.
Japanese Prime Minister Junichiro Koizumi is anxious that a deal bearing the name of Japan's ancient capital, Kyoto, should not fail, but he has also made plain he does not wish to see his powerful American ally isolated.
He had voiced caution on the prospects for two weeks of meetings in Bonn reaching a final deal, which would have to include nitty-gritty details of how pledges to cut greenhouse gas emissions would be put into effect and monitored.
But his environment minister, Yoriko Kawaguchi, said in the former West German capital that Tokyo would not hold up implementation of the pact next year if Bush could not be won over.
"It is very important for all the countries to combat global warming under one rule and therefore to have the United States participate is the best scenario," she said.
"At the same time, we do not have any intention of delaying the international process that is going on."
Having much of the rest of the industrial world - without the United States - commit itself to cutting its emissions of carbon dioxide and other greenhouse gases to 5.2 per cent below 1990 levels by 2012 would be second-best, Kawaguchi said.
Bush says the Kyoto pact is based on dubious science and would hurt the economy.
US delegates were keeping a low profile this week as officials from the 180-odd countries present discussed technical issues.
Much of the high-level arm twisting may happen at the Group of Eight summit in Genoa, Italy, this weekend, when Bush will meet German, French, British, Italian, Canadian and Russian leaders.
- REUTERS
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Intergovernmental Panel on Climate Change (IPCC)
United Nations Environment Program
World Meteorological Organisation
Framework Convention on Climate Change
Executive summary: Climate change impacts on NZ
IPCC Summary: Climate Change 2001