The Islamic State of Iraq and the Levant (ISIL) has released a video purportedly showing the killing of Japanese hostage Kenji Goto, a monitoring service has said.
The video, released online by the armed group on Saturday, shows the journalist who was abducted while reporting on Syria's civil war last year being beheaded with a knife by a black-clad masked fighter.
Japan on Sunday said it was "highly probable" the video is authentic.
When asked if the man in the video was 47-year-old Goto, government spokesman Yoshihide Suga replied: "We think so."
Addressing Japanese Prime Minister Shinzo Abe, the ISIL member in the video said: "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."
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Abe addressed the nation shortly after the release of the footage and appeared visibly shaken as he condemned the "horrendeous" and "despicable" killing.
"Japan will not be defeated by terrorism," he said, adding that the government had tried its best to save the hostage.
US President Barack Obama also condemned "the heinous murder" of Goto and praised Japan.
"We applaud Japan's steadfast commitment to advancing peace and prosperity in the Middle East and globally, including its generous assistance for innocent people affected by the conflicts in the region," Obama said in a statement on Saturday.
ISIL continue to hold another hostage, Jordanian pilot First Leutenant Moaz al-Kasasbeh.
In an interview with Al Jazeera on Saturday, Kasasbeh's mother urged ISIL to release her son.
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"I call upon the Islamic State group, I appeal to our brothers among them, from one Muslim addressing another, from a distressed mother's heart who is in sorrow for missing her beloved son," said Om Jawad al-Kasasbeh
"I appeal to you tell us about Moaz's wellbeing and I ask you to be kind in how you treat him and to release him. I beg you, I ask you to show him mercy."
'Chained by the wrist'
Goto was a freelance journalist who had reported from war zones for more than 20 years, in Africa, Afghanistan and the Middle East.
He travelled to Syria, at least in part, to attempt a rescue of his friend Haruna Yukawa, who was captured by ISIL in August.
Haruna was a self-styled military consultant who went to Syria to set up a security company, despite little experience.
In October, Goto was captured by ISIL and was held captive alongside his friend. ISIL demanded a $200m ransom for the two men.
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Last week, the armed group released a video with an image of Goto , chained by the wrist and with a photograph of Yukawa in his hand, with audio of Yukawa being killed.
The video said Goto had "only 24 hours left to live" and Kasasbeh "even less".
The footage later gave a deadline of sunset on Thursday, for Goto to be freed in return for Jordan releasing the Iraqi woman Sajida al-Rishawi.
ISIL has beheaded and shot dead hundreds of captives - mainly Syrian and Iraqi soldiers - during its sweep across the two countries, and has celebrated its mass killings in extremely graphic videos.
The group also beheaded US hostages James Foley and Peter Kassig, Israeli-American Steven Sotloff and British captives David Haines and Alan Henning.
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Source: Al Jazeera and agencies