Prime Minister Shinzo Abe on Sunday denounced as "heinous and despicable" the apparent beheading of a second Japanese hostage by the Islamic State group, as global leaders denounced the militants.
IS claimed in a video released online Saturday that it had killed 47-year-old Kenji Goto - the second beheading of a Japanese hostage in a week - but made no mention of a captured Jordanian pilot it had also threatened to kill.
Jordan "will do everything it can" to save the pilot's life and secure his release, government spokesman Mohammed al-Momeni told the official Petra news agency in Amman.
Goto, a respected war correspondent, is seen in an orange outfit - similar to those worn by Guantanamo Bay inmates - kneeling next to a standing masked man dressed head-to-toe in black with his face covered.
The man, who speaks with a British accent, appears to be the same IS militant who has featured in previous execution videos.
He directly addresses Abe, saying the killing was the result of "reckless" decisions by the Japanese government -- a possible reference to aid money that Tokyo gave to help refugees fleeing IS-controlled areas in Syria and Iraq -- and would mark the beginning of a "nightmare for Japan".
The brief video, whose content has not been verified, ends with the image of a body and a decapitated head on top of it.
"After an extensive review, we believe it's highly probable" the video is authentic, government spokesman Yoshihide Suga told reporters.
Japan's premier, who appeared on the verge of tears, pledged not to back down and said his government would increase humanitarian aid to the Middle East.
"I am extremely angry about these heinous and despicable terrorist acts. We will never forgive terrorists," he told a crush of reporters at his office.
"We will cooperate with the international community to make them atone for their crimes."
'Chills down my spine'
"I can't find the words to describe how I feel about my son's very sad death," a sobbing Junko Ishido, Goto's mother, told reporters.
His brother Junichi Goto said he had been holding out hope, "but that's not possible any more".
Officially pacifist Japan has long avoided getting embroiled in Middle East conflicts and is rarely the target of violence linked to religion. So the hostage crisis has been especially shocking for the country.
Many braved Tokyo's chilly streets to pick up the Yomiuri newspaper's special supplement about the Goto video.
"It's scary -- they (the militants) are saying they'll target Japanese people now," said 21-year-old university student Kyosuke Kamogawa.
"That sends chills down my spine."
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 said the "barbaric murder... underscores the violence that so many have been subjected to in Iraq and Syria".
Paris and London also denounced the video, with British Prime Minister David Cameron saying it was "a further reminder that (IS) is the embodiment of evil, with no regard for human life".
The militant group has imposed a brutal version of Islamic law in territory it controls in Syria and Iraq. It has murdered both locals and foreigners, including two US journalists, an American aid worker and two British aid workers.
The latest video came after Japan, which had leaned heavily on key regional ally Jordan for help, said negotiations to win Goto's release in a prisoner exchange had stalled.
IS had vowed to kill Goto and Jordanian pilot Mu'ath al-Kassasbeh by sunset Thursday unless Amman handed over an Iraqi woman who is on death row for her part in an al-Qaeda in Iraq attack in the capital that killed 60 people in 2005.
"The government has been working with the utmost efforts on the issue - I deeply regret that this is the result," Abe said.
But "Japan will never yield to terrorism... (and) is firmly resolved to fulfil its responsibility in the international community's fight against terrorism".
'Proof of life'
Last week, IS claimed responsibility for the beheading of another hostage, self-described contractor Haruna Yukawa, after the expiration of a 72-hour deadline during which it had asked Tokyo to pay a $200 million ransom.
It was the same sum that Tokyo had pledged to the region in non-military aid.
Jordan has demanded evidence that its pilot, who crashed in Syria on 24 December, was still alive before freeing would-be suicide bomber Sajida al-Rishawi.
The pilot's father Safi Kassasbeh begged Amman to save his son's life "at any price".
Goto's wife Rinko broke her silence last week to plead for her husband's return.
The couple had a second child just weeks before Goto left for Syria late last year, in a bid to find his friend Yukawa. He was later captured himself.
Diplomats from the Council of Arab Ambassadors to Japan were to meet Foreign Minister Fumio Kishida later Sunday.
The Council "deplores that such a barbaric act was committed...in the name of Islam", it said in a statement.
