Pope Francis said the world was facing a "grave environmental crisis" as he arrived in Kenya on Wednesday on a landmark Africa trip just days before a crucial UN summit aimed at curbing climate change.

"The grave environmental crisis facing our world demands an ever greater sensitivity to the relationship between human beings and nature," he told Kenyan political leaders in the capital Nairobi, shortly after arriving in the east African country.

"There is a clear link between the protection of nature and the building of a just and equitable social order."