Pope Francis has arrived on the Greek island of Lesbo, turning the world's attention to the front line of Europe's migrant crisis, which has claimed hundreds of lives in the past year.

Francis, leader of the world's 1.2 billion Roman Catholics, was scheduled to spend about six hours on the small Aegean island. Based on his schedule, he was to meet 250 refugees and have lunch with eight of them.

Hundreds of people have died making the short but precarious crossing from Turkey to the Lesbos shores in inflatable dinghies in the past year, and the island is full of unmarked graves.

"This is a trip that is a bit different than the others...this is a trip marked by sadness," Francis told reporters on the plane taking him to Lesbos.

"We are going to encounter the greatest humanitarian catastrophe since World War Two. We will see many people who are suffering, who don't know where to go, who had to flee.

"We are also going to a cemetery, the sea. So many people died there ... this is what is in my heart as I make this trip."

With Ecumenical Patriarch Bartholomew, leader of the world's Orthodox Christians, and Greek Prime Minister Alexis Tsipras, Francis will visit Moria, a sprawling, fenced complex holding more than 3,000 refugees.

"This is an island which has lifted all the weight of Europe upon its shoulders," Tsipras told Francis at Lesbos airport, where a red carpet was rolled out for the pontiff's arrival.

Greek state TV reported Francis was planning to take 10 refugees back with him to the Vatican, eight of them Syrians.

Aid organisations have described conditions at Moria, a disused army camp, as appalling.

Journalists have no access to the facility on a hillside just outside Lesbos's main town of Mytiline, but aid workers said walls were whitewashed, a sewer system fixed and several dozen migrants at the overcrowded facility were transferred to another camp, which the pope will not visit.

Aid organisations say queues for food are long and people often wait for an hour or more.

Saturday's encounter with refugees would be 'no frills' and the religious leaders would eat the same food as everyone else at the camp, an official at the camp told Reuters.

Greek media reported Saturday's fare would be risotto and mushrooms and olives, and halva, a sweetbread made with sesame and honey.