On his four-day private visit to China, Hatoyama told reporters in Beijing on Wednesday, "The Japanese government says there are no territorial disputes (between the two countries). But if you look at history, there is a dispute."
The remarks contradict his own government's position of indisputable territorial sovereignty over the islands that it calls Senkaku and that China calls Diaoyu.
"If his (Hatoyama's) remarks have been politically used by China, I'm unhappy," Japanese Defense Minister Itsunori Onodera said on television Thursday. "At that moment, the word of 'traitor' arose in my mind."
Dangerous waters: Behind the islands dispute The day after his controversial remarks, Hatoyama, 66, and his wife visited the Nanjing Memorial, which is for the estimated 300,000 people killed in a 1937 massacre by Japanese forces.
He is the third former Japanese prime minister to visit the memorial, following predecessors Toshiki Kaifu and Tomiichi Murayama. The tribute for Chinese victims stands in contrast to visits by Japanese officials, including Prime Minister Shinzo Abe and predecessor Junichiro Koizumi, to the controversial Yasukuni Shrine in Tokyo, which is dedicated to Japan's war dead, including war criminals.
"In the eyes of the Chinese public, (Hatoyama's) visit is very valuable and undermines those in China who argue that all Japanese suffer from amnesia about wartime misdeeds," said Jeff Kingston, director of Asian studies at Temple University Japan. "I think this mission is an effort by him to introduce a different tenor into bilateral relations, to show it's not all about saber-rattling," he added.
Chinese media extensively covered Hatoyama's "apology for Japan's wartime crimes," with pictures of the Hatoyamas bowing and paying silent tribute at the site.
On social media, the visit triggered wide discussions. According to an online poll by Phoenix Online (iFeng), 80% of the more than 222,000 people who voted said Hatoyama's visit did not have much political significance, as compared with German Chancellor Willy Brandt's kneeling before the Warsaw Ghetto Memorial in 1970.
In another question, nearly two-thirds of 140,000 people surveyed said the visit by a former leader wouldn't set an example for a fellow Japanese politician.
CCTV commentator Yang Yu, however, praised Hatoyama and urged the Chinese to "remember the unusual kindness due to its scarcity," saying via Weibo, the microblogging site, "We have reprimanded Japan too many times for not acknowledging the massacre."
The official account from Xinhua, China's state-run news agency, warned that the nationalism of people who "scold any Japanese they see ... is in fact leading the country to distress."
An editorial by the government-run Global Times said that "China shouldn't change its policy to Japan just because Hatoyama, a politician currently out of office, gave a few words of friendship."
Grievances over World War II atrocities added fuel to violent anti-Japanese protests in China in September, particularly on the anniversary of the 1931 Japanese invasion of China.
And it is not a coincidence, Kingston said, that a Chinese plane entered airspace over the disputed islands -- prompting Japan to scramble fighter jets -- on December 13, the 75th anniversary of the massacre. It was the first time that the territorial dispute involved planes.
"The next day on the front page of the newspapers were the images of the (Nanjing) Memorial ceremony and the planes," Kingston said.
The dispute over the islands stems from 1895, when, at the end of the Sino-Japanese war, Japan annexed them. China has said that the islands have been its territory for the last five centuries.