Pope Francis delivers his speech with his cape blown by wind, at the Martyrs' Monument at Nishizaka Hill Sunday, Nov. 24, 2019, in Nagasaki, Japan. (AP Photo/Gregorio Borgia)

Tokyo - Pope Francis on Sunday paid tribute to the victims of US nuclear bomb attacks on two Japanese cities at the close of World War II and described the use of atomic energy for purposes of war as a "crime."

After arriving in Hiroshima in the evening, Francis laid flowers at the arch-shaped Hiroshima Peace Memorial near ground zero and lit a candle to pray for peace.

The 82-year-old pope listened to the accounts of two atomic bomb survivors before delivering his speech.

"I felt a duty to come here as a pilgrim of peace," said the pope, who kicked off a four-day tour to Japan on Saturday.

The atomic bombing on Hiroshima on August 6, 1945, killed tens of thousands of residents instantly, and by the end of the year, some 140,000 in total had died.

"Here, in an incandescent burst of lightning and fire, so many men and women, so many dreams and hopes, disappeared, leaving behind only shadows and silence," the pope said.

"From that abyss of silence, we continue even today to hear the cries of those who are no longer."

The pope said "the use of atomic energy for purposes of war is today, more than ever, a crime not only against the dignity of human beings but against any possible future for our common home."

Earlier in the day, the pope visited Nagasaki, the other city struck by an atom bomb in 1945.

"This place makes us deeply aware of the pain and horror that we human beings are capable of inflicting upon one another," Francis said at the Atomic Bomb Hypocenter Park in rainy Nagasaki.

The nuclear attack on Nagasaki killed about 74,000 people, including more than 8,000 Catholics. It came three days after the first bombing on Hiroshima. 

"Here in this city which witnessed the catastrophic humanitarian and environmental consequences of a nuclear attack, our attempts to speak out against the arms race will never be enough," the pope said in Nagasaki.  

"One of the deepest longings of the human heart is for security, peace and stability. The possession of nuclear and other weapons of mass destruction is not the answer to this desire; indeed they seem always to thwart it," the pope said.

"Peace and international stability are incompatible with attempts to build upon the fear of mutual destruction or the threat of total annihilation."

The pope delivered a "clear message calling for a nuclear-free world" at the ground zero, which is an important place for Nagasaki, the city's mayor Tomihisa Taue said, referring to the Atomic Bomb Hypocenter Park.

His words also encouraged atomic bomb survivors and Nagasaki citizens, Taue said.

The Holy See was among the first to sign and ratify the Treaty on the Prohibition of Nuclear Weapons, which was adopted by 122 countries in July 2017. However, it has yet to be in force as it has not been ratified by the required 50 countries.

Anti-nuclear activists urged Japan, the only country to have suffered atomic bombings, to sign and ratify the treaty as soon as possible. But Prime Minister Shinzo Abe's government has so far refused to do so as the country is under the US nuclear umbrella.

"Convinced as I am that a world without nuclear weapons is possible and necessary, I ask political leaders not to forget that these weapons cannot protect us from current threats to national and international security," the pope said.

"We need to ponder the catastrophic impact of their deployment, especially from a humanitarian and environmental standpoint, and reject heightening a climate of fear, mistrust and hostility fomented by nuclear doctrines."

The pope also held a mass at the city's baseball stadium in the afternoon with around 30,000 people in attendance.

While only 0.35 per cent of Japan's 127 million people are Catholic, Christianity first took root in Nagasaki.

On Monday, Francis is scheduled to hold a meeting with Emperor Naruhito, who ascended to the throne on May 1, and Abe in Tokyo.

The pope is also to meet victims of the 2011 earthquake, tsunami and nuclear disaster in north-eastern Japan.

His tour marks the first papal visit to Japan in 38 years. In 1981, Pope John Paul II travelled to Hiroshima, Nagasaki and Tokyo. 

dpa