Russian authorities have failed to take into account the environmental impact of building work ahead of the 2014 Winter Olympics, the UN has said.
The UN report comes after leading environmental organisations in Russia withdrew their co-operation, saying their concerns were being ignored.
Authorities say the Games will be green and have accused environmentalists of trying to derail the event.
The Games will be held in Sochi, a southern Russian Black Sea resort.
It is a sensitive region - home to a national park and a huge nature reserve designated as a world heritage site.
Relations between environmental organisations and the authorities responsible for the massive Olympics building programme have deteriorated sharply in recent months.
The World Wide Fund for Nature (WWF) recently withdrew all co-operation.
It is warning that irreversible damage has already been done to the environment, including the pollution of important rivers and the felling of rare species of trees.
Now the United Nations Environment Programme has added its voice.
It says the authorities need to carry out a full assessment of the overall impact the Olympics building work is having on the ecosystem.
It also calls for a strong monitoring programme and has offered to help mediate between the environmentalists and the authorities.
Much is at stake. This is the first time Russia is hosting the Winter Olympics and it is a pet project of Prime Minister Vladimir Putin.