ATHENS - U.N. Secretary-General Ban Ki-moon says global talks on climate change have reached a "critical period" ahead of next month's key climate conference in Copenhagen, Denmark.

Ban says the threat of environmental refugees from developing countries is adding pressure for agreement to be reached at the United Nations conference on Dec. 7-18.

The U.N. chief spoke at an international conference on immigration Wednesday in Greece, where he began a two-day official visit.