"They're rattled by him and for good reason," Obama said. "Because a lot of the proposals that he's made display either ignorance of world affairs or a cavalier attitude or an interest in getting tweets and headlines instead of actually thinking through what is required to keep America safe."
Asked about the continued divisiveness on display on the Democratic side of the 2016 race, Obama said people get "grumpy" during the primaries, including his own back in 2008, but will ultimately come together and argued that Hillary Clinton and Bernie Sanders largely agree on the policy front, but differ on tactics.
Obama also largely deflected questions about Clinton's use of a private email server, saying he's addressed the topic previously and the questions would be better put to the campaign, CNN reported.
Thursday morning saw the leaders of Japan, the US, Britain, Italy, Germany, France, and Canada gather in the grounds of the Ise-Jingu shrine, a 2,000-year-old temple in central Japan.
Obama also said he is going to Hiroshima Friday to underscore the "very real risks" of nuclear weapons and the "urgency that we all should have."
He will become the first US President to visit the Japanese city where the first atomic bomb was dropped.
Obama told reporters that the dropping of the bomb was an "inflection point in modern history" and is something "all of us have had to deal with in one war or another."
He added that the "backdrop of a nuclear event remains something that, I think, presses on the back of our imaginations."