Japan says 'highly probable' IS beheading video is authentic

"After an extensive review, we believe it's highly probable" the video is real, government spokesman Yoshihide Suga said in response to a reporter's question about its authenticity. When asked if the man in the video was 47-year-old journalist Kenji Goto, Suga replied: "We think so".

The video did not mention the fate of a Jordanian pilot also being held by IS, whom the militant group has threatened to kill. In the video, Goto is seen in an orange outfit - similar to those worn by Guantanamo Bay inmates - kneeling next to a standing masked man who speaks with a British accent and blames the Japanese government for his "slaughter".

The man, dressed head-to-toe in black with his face covered, appears to be the same IS militant who has featured in the group's previous execution videos. The executioner addresses Japanese Prime Minister Shinzo Abe, saying the murder was the result of "reckless" decisions by the Japanese government and would mark the beginning of "the nightmare for Japan".

Abe vowed to "never forgive terrorists" after news of the video broke early Sunday morning in Japan. "I am extremely angry about these heinous and despicable terrorist acts. We will never forgive terrorists," the premier, who appeared on the verge of tears, told reporters. "We will cooperate with the international community to make them atone for their crimes."

Goto's distraught mother said she "can't find the words" to describe her son's death. "It's deplorable, but Kenji is gone," a sobbing Junko Ishido told reporters. "I can't find the words to describe how I feel about my son's very sad death."

Goto's brother Junichi Goto said he had been holding out hope, "But that's not possible anymore," he was quoted as saying by public broadcaster NHK.

US President Barack Obama led international condemnation of the "heinous murder". "Through his reporting, Mr Goto courageously sought to convey the plight of the Syrian people to the outside world," Obama said.

A spokesman for UN chief Ban Ki-moon also condemned the "barbaric murder", and said the death "underscores the violence that so many have been subjected to in Iraq and Syria".

British Prime Minister David Cameron denounced the apparent killing as "a further reminder that ISIL (IS) is the embodiment of evil, with no regard for human life." French President Francois Hollande also condemned the "brutal murder".

The apparent execution came after Japan said negotiations to win Goto's release in a prisoner exchange had stalled. IS had vowed to kill Goto and Jordanian pilot Maaz al-Kassasbeh by sunset Thursday unless Amman handed over an Iraqi female militant.

On Saturday morning Abe had renewed orders for officials to maintain close cooperation with Jordan in a bid to secure Goto's release. "The government has been working with the utmost efforts on the issue -- I deeply regret that this is the result," Abe told reporters on Sunday.

But "Japan will never yield to terrorism... (and) is firmly resolved to fulfil its responsibility in the international community's fight against terrorism".