Nairobi (AFP) - 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."

Francis has taken on a vocal role in the fight against climate change, injecting a moral voice into a debate usually dominated by economic and political concerns.

His remarks come on the eve of a United Nations climate conference in Paris which will gather more than 145 world leaders for a 12-day summit aimed at securing a climate rescue pact.

The 78-year-old pontiff said environmental concerns should be at the forefront of "responsible" economic development.

"Kenyan people... are known for a culture of conservation, which does you honour," he said.

"We have a responsibility to pass on the beauty of nature in its integrity to future generations, and an obligation to exercise a just stewardship of the gifts we have received. These values are deeply rooted in the African soul.

"In a world which continues to exploit rather than protect our common home, they must inspire the efforts of national leaders to promote responsible models of economic development."

The pontiff also drew a "clear link" between climate concerns and social justice.

"In effect, there is a clear link between the protection of nature and the building of a just and equitable social order," he said. "There can be no renewal of our relationship with nature, without a renewal of humanity itself."

- A firm stance -

The leader of the world's 1.2 billion Catholics took a firm stance on environmental issues earlier this year, issuing a lengthy 'encyclical' which said climate change was primarily man-made and one of the main challenges facing humanity.

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The text called for action to phase out fossil fuels but also pointed the finger at big industry, and blamed first-world countries for failing to act.

He is also expected to address climate issues again on Thursday when he visits the Nairobi headquarters of the UN's Environment Programme (UNEP) and Human Settlements Programme (UN-Habitat).

Earlier this year, the Argentine pope said the world was at a "critical moment" in the fight against climate change, describing it as "a problem which can no longer be left to a future generation."

His environmental message is likely to resonate in Africa where climate change is already being felt and where the poaching of endangered species such as elephants and rhinoceroses is rampant.

President Uhuru Kenyatta echoed those concerns during a welcome address to the pope on Wednesday in which he warned that "climate change, greed and poaching" threatened Kenya's natural heritage.

Francis is not the first pope to be interested in green matters. Since the early 1960s, popes have paid increasing attention to the relation between pollution, waste and poverty, with Paul VI warning in 1971 of the potential for ecological catastrophe and need for change in human behaviour.