SEOUL -- With a U.S. delegation in attendance for the first time, a Japanese ceremony Friday commemorating history's first atomic bomb attack coincided with renewed hopes that President Obama will visit Hiroshima or Nagasaki, something no sitting U.S. president has done.

In what Japanese officials described as a significant first step, U.S. Ambassador John Roos represented Washington at the anniversary event at Hiroshima's Peace Memorial Park, 65 years after a U.S. bomb leveled the city, killing roughly 140,000. By several measurements, this year's ceremony was the biggest yet: Representatives from a record 74 countries showed up. Britain and France participated for the first time, as did U.N. Secretary General Ban Ki-moon.

(Atomic bomb images from Life magazine)

In a speech, Japanese Prime Minister Naoto Kan described a "new momentum" toward nuclear disarmament and nonproliferation. He attributed that to Obama, who in April 2009 spoke of a "world without nuclear weapons," and who four months ago hosted world leaders at a nuclear summit in Washington.

U.S. unease with the Hiroshima and Nagasaki commemorations has long prodded at a sensitive spot in an otherwise enduring alliance. And despite recent diplomatic wrangling over the relocation of a U.S. air base on Okinawa, Obama remains a popular figure in Japan. According to the Associated Press, Obama T-shirts were on sale Friday at the Peace Park's museum.

Obama has said he is open to visiting Hiroshima or Nagasaki. However, he declined an invitation last year, citing his busy schedule. His planned visit to Japan in November is likely to prompt another emphatic invitation -- especially at a time when the average atomic bomb survivor is 76.

(Photos from the Hiroshima ceremony)

"It is up to the United States to ultimately decide on it," Kan said, "and I would like to refrain from making any remarks that would lead to prejudgment."

Roos had visited Hiroshima once before, although not on the Aug. 6 anniversary. Roos did not speak at the event, but the U.S. Embassy in Japan released a statement in which he called for continued work toward nuclear disarmament "for the sake of future generations."

The statement added that Japan and the United States "share a common goal of advancing President Obama's vision of a world without nuclear weapons."

The Hiroshima anniversary comes a time when Japan faces its own internal debate about the need for protection under the U.S. nuclear umbrella. The latest provocations from nuclear-capable North Korea have caused a spike in regional tensions. And although Japan has extensive economic ties with China, Tokyo is also wary of the rapid Chinese military expansion.

Hiroshima's Peace Memorial Park, with the ruined building known as the A-Bomb Dome in the distance, served as the usual stage Friday for school choirs and bell-ringing -- as well as an 8:15 a.m. moment of silence.

But during a speech at the memorial, Hiroshima Mayor Tadatoshi Akiba urged the Japanese government to abandon its reliance on the U.S. nuclear umbrella. He also suggested that Japan's three non-nuclear principles -- currently a resolution -- should be turned into law. (Japan stands against the production, possession and introduction of nuclear weapons.)

Kan, speaking in the morning, had voiced optimism about denuclearization.

"I firmly believe that Japan, as the only country to have experienced nuclear devastation in war, has a moral responsibility to lead actions toward realizing 'a world without nuclear weapons,' " he said. In a subsequent news conference, though, Kan acknowledged that Japan still depends on the nuclear umbrella. "Nuclear deterrence continues to be necessary for our nation," he said.

Three days after the bombing of Hiroshima, a bomb dropped on Nagasaki killed an estimated 80,000 people. Japan surrendered six days later, ending World War II.

While no sitting American president has been to either city, Jimmy Carter visited Hiroshima in 1984 after he had left office. No sitting Japanese prime minister has visited Pearl Harbor, which was attacked by Japanese forces on Dec. 7, 1941.

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