The U.N. chief has praised Pope Francis for framing climate change as an urgent moral imperative, saying his upcoming encyclical combined with a new round of U.N. climate talks in Paris provide an "unprecedented opportunity" to create a more sustainable future for the planet.

Secretary-General Ban Ki-moon opened a Vatican conference on the environment Tuesday that is a key part of the Holy See's rollout of Francis' eagerly awaited encyclical, which is expected in June.

Ban told Vatican officials and faith leaders from several different traditions that there is large consensus between religious leaders and scientists that climate change is real, that humans must change their behavior to stop it and that "protecting our environment is an urgent moral imperative and sacred duty for all people of faith and conscience."