The chief of the U.N. Environment Program urged Japan to keep playing a leadership role in the fight against global warming, although it faces a challenge caused by Fukushima nuclear disaster, which left most of its atomic plants offline.

"The world has in a sense believed in Japan as a leader and a pioneer. That is a brand many would like Japan to retain," UNEP Executive Director Achim Steiner said in an interview.

"There has been a tremendous disaster. That is very challenging for any country and there's a lot of sympathy," he said. "But I think the message hopefully from Japan is, 'Look, we have a short-term challenge, but we have a full commitment to our long-term goal as a leader in addressing climate change.' "