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Obama told reporters at the Group of 7 Summit in Japan that Trump displays ignorance of world affairs and a cavalier attitude.

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"I think it's fair to say they're surprised by the Republican nominee," Obama said in Ise-Shima, Japan. 

"They are not sure how seriously to take some of his pronouncements, but they're rattled by him, and for good reason," Obama added.

"A lot of the proposals he's made display either ignorance of world affairs, or a caviler attitude or an interest in getting tweets and headlines instead of actually thinking through what is required to actually keep America safe and secure and prosperous and what's required to keep the world on an even keel."

Obama is in Japan as part of a historic week-long trip through Asia aimed at strengthening U.S. ties with the region. He will visit Hiroshima on Friday, making him the first sitting American president to visit the site of the world's first atomic bombing.

—Updated at 8:20 a.m.