MARRAKECH, Morocco?Technical and legal experts began work Tuesday on the fine points of rules to implement a global treaty on controlling climate change. Environmentalists expressed concern that some industrial countries would backtrack on their support.

Australia, Canada, Japan and Russia were singled out for worry by the environmental group Greenpeace. With the United States already opting out, a refusal of the four to ratify the treaty would threaten its prospects.

The two-week conference which opened Monday culminates a four-year effort to draft binding regulations on limiting greenhouse gas emissions that contribute to global warming.

Nearly 4,000 delegates from 163 countries and nongovernment organizations are attending. The U.S. delegation is observing rather than negotiating.