U.N. Secretary—General Ban Ki—moon called for the abolishment of nuclear weapons on Thursday during a visit to Nagasaki, one of two Japanese cities devastated by U.S. atomic bombs in the closing days of World War II.

Ban toured the Atomic Bomb Museum and met with six survivors during his visit, the first by a U.N. chief to Nagasaki. More than 70,000 people were killed when the U.S. bombed the southern Japanese city on Aug. 9, 1945.

“My visit here has strengthened my conviction that these weapons must be outlawed, either by a nuclear weapons convention or by a framework of separate mutually reinforcing instruments,” Mr. Ban said in a speech at Nagasaki’s Urakami Cathedral.

The cathedral, just 2,000 feet (600 meters) from the bomb’s hypocenter, was completely destroyed and was rebuilt along with much of the city after the war.

Mr. Ban said nations must work together to create a world free from nuclear weapons.

“The only way to ensure that such weapons will never again be used is to eliminate them all,” he said. “There must be no place in our world for such indiscriminate weapons.”

Mr. Ban is to visit Hiroshima on Friday to attend the 65th anniversary of the U.S. atomic bombing of that city on Aug. 6, 1945. About 140,000 people were killed or died within months in Hiroshima. Japan surrendered on Aug. 15, ending World War II.

U.S. Ambassador John Roos will also attend the ceremony in Hiroshima, becoming the first representative sent by Washington to the annual commemoration.

Nuclear powers France and Britain will also send representatives to the Hiroshima ceremony for the first time.

Former President Jimmy Carter visited Hiroshima’s Peace Museum in 1984, after he was out of office. The highest—ranking American to visit while in office is House Speaker Nancy Pelosi, who went in 2008.

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