Kunio Hoshi was shot to death by unidentified assailants in northern Bangladesh on Saturday. Bangladesh's government rejected a statement by the Islamic State group claiming responsibility for the attack.
"Such a dastardly act should never be repeated. I feel outraged by such an act," Yoshihide Suga, Japan's chief Cabinet secretary, told reporters in Tokyo today.
He extended condolences to Hoshi's family, and said the Japanese government was seeking a full investigation into the case.
The Islamic State group issued a statement claiming responsibility for Saturday's attack, according to the SITE Intelligence Group, which monitors jihadi postings online.
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The report could not be independently confirmed. The Islamic State also claimed responsibility for the killing of an Italian aid worker last week in Bangladesh's capital.
"Oh, it's absolutely rubbish, there is no IS in the country, no way," Bangladeshi Home Minister Asaduzzaman Khan told The Associated Press yesterday.
"The claims are fishy and we are examining," he said.
Khan's view was echoed by Prime Minister Sheikh Hasina, who also dismissed the Islamic State's claims.
"Someone will post something online ... Why should we accept that unless we prove that? We cannot accept that," she told reporters.