Pope Francis has said he wanted to offer "spiritual and material" support on his first tour of Africa, where he will address a fast-growing Catholic congregation and seek to heal divisions between Christians and Muslims.

The pope arrived in Kenya before travelling to Uganda, another nation targeted by Islamist militant attacks. After that he visits the Central African Republic, a country torn apart by Muslim-Christian strife.

"I am going with joy to Kenya, Uganda and the brothers of the Central African Republic," he told reporters aboard his flight to Nairobi. "Let's hope this trip brings better fruit, both spiritual and material."

Millions of Christians - Catholics and others - are expected to turn out for public celebrations of Mass, presenting a challenge for national security forces to keep the pontiff and the huge crowds safe.

Kenya has been the target of a spate of attacks by Somalia's Islamist group al Shabaab in the past two years that have killed hundreds of people. In 2013, an assault by al Shabaab gunmen on a Nairobi shopping mall killed 67 people.

Potentially the most hazardous stop may be the third leg of the trip to the Central African Republic. Dozens of people have been killed there since September in violence between mostly Muslim Seleka rebels and Christian anti-balaka militias.

Asked if he was nervous, the pope brushed off concerns about the trip with a joke: "To tell you the truth, the only thing I'm concerned about is the mosquitoes. Did you bring your spray?"