A video purporting to show the execution of hostage Kenji Goto was posted early Sunday morning by the Islamic State of Iraq and the Levant (ISIL) militant group.
Prime Minister Shinzo Abe strongly condemned the apparent killing and reiterated Japan's stance against terrorism, saying, "Japan will never give in to terrorism."
"As a government, we have pursued every possible means to save their lives. We feel greatest sorrow and profound grief. I am infuriated by these inhumane and despicable acts of terrorism, and resolutely condemn these impermissible and outrageous acts," Abe said in a statement.
"We will further expand our humanitarian assistance in the Middle East in areas such as food and medical care." The statement was also released in English and Arabic.
In the 67-second-long video, a masked ISIL militant says in English: "Abe, because of your reckless decision to take part in an unwinnable war, this knife will not only slaughter Kenji, but will also carry on and cause carnage wherever your people are found.
"So let the nightmare for Japan begin."
The government called a meeting of ministers concerned at the Prime Minister's Office on Sunday morning.
"Taking everything into account, [the image] is highly likely to be that of Mr. Goto," Chief Cabinet Secretary Yoshihide Suga said at a press conference shortly before noon.
The government learned of the video at about 5 a.m. and contacted Goto's family.
Suga arrived at the Prime Minister's Office about half an hour later to gather and analyse information with government officials including Deputy Chief Cabinet Secretary Kazuhiro Sugita and Yasuhiko Nishimura, deputy chief cabinet secretary for crisis management. Abe joined them there a few minutes later.