US President Barack Obama strongly condemned the killing of one of two Japanese hostages by the Islamic State group, announced in a video that has not yet been officially authenticated.
Obama's comments, echoed Saturday by British Prime Minister David Cameron who condemned the group's "murderous barbarity," suggested that Western intelligence had authenticated the video posted online.
The nearly three-minute recording shows a still image of Kenji Goto holding an apparent photograph of Haruna Yukawa's slain body, with an audio recording in which Goto spoke of the IS group's demand for a prisoner exchange to guarantee his release.
It was not posted on any of the group's official channels and it does not bear their black and white flag. The purported execution of Yukawa is also not shown.
The US-based SITE monitoring service said it could confirm the video was in fact released officially by IS, citing its research and understanding of the online infrastructure of the group and other militants.
"The video was made in a different style than the other beheading videos," SITE director Rita Katz said, "seemingly rushed and even lacking the usual attribution to al-Furqan Media Foundation, a primary media arm of the group."
- 'Intolerable violence' -
With a "speechless" Prime Minister Shinzo Abe condemning the militants' "intolerable violence," Japan was meanwhile scrambling to verify the video.
Following a predawn emergency meeting of senior ministers, Abe demanded the immediate release of Goto and vowed his government would do "its utmost" to free him.
"The pain the family is feeling must be beyond what we imagine. This is an act of terrorism and intolerable violence," Abe said.
Obama said the United States "strongly condemns the brutal murder" of Yukawa, and stands "shoulder to shoulder with our ally Japan."
Several supporters of the IS group on social media channels have contested the veracity of the video, while Syrian Observatory for Human Rights director Rami Abdel Rahman said negotiations for the two men's release are still ongoing.
"The intelligence community is working to confirm its authenticity," US National Security Council deputy spokesman Patrick Ventrell said in a statement.
If Yukawa's death is confirmed, it would mark a grave turn of events for Japan, which has been on edge since the Islamic State group released a video on Tuesday demanding a $200 million ransom within 72 hours.
There had been no news of the fate of freelance journalist Goto or self-employed contractor Yukawa since the deadline expired at 0550 GMT on Friday.
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The jihadist group, which rules large swaths of Iraq and Syria under a strict form of Islamic law, has murdered five Western hostages since August last year, but this is the first time it has threatened Japanese captives.
- Emotional appeal -
Junko Ishido, Goto's mother, had launched an emotional appeal begging for mercy for her son.
"I say to you people of the Islamic State, Kenji is not your enemy. Please release him," she said.
Japanese officials have repeatedly said they are trying to make contact through various channels.
Abe advisor Yosuke Isozaki reportedly said there had been some "indirect" communication with the militants, but "nothing direct."
Tokyo has little diplomatic leverage in the Middle East, but local media say Abe may try to use his close relationship with Turkey's President Recep Tayyip Erdogan to rescue the hostages.
Japan's Yomiuri Shimbun newspaper reported that Jordan has also tried to contact the Islamic State through influential religious leaders in Amman.
The Islamists linked their $200 million ransom demand to the amount Abe said he would earmark to help countries dealing with the influx of refugees fleeing fighting between IS militants and regular forces.
Tokyo is under pressure from Britain and the United States to stand firm on the group's cash demands, as both countries have a policy of never paying ransoms.
Abe vowed that "the government of Japan will never give in to terrorism."
Broadcaster NHK on Friday quoted a "public relations" official from the Islamic State group describing the Japanese as "infidels" for supporting efforts to stop them.
The Japanese media has rallied around Goto, a respected and experienced war reporter whose work has appeared on domestic television channels.
In video footage he filmed around the time he entered Syria, he holds identification papers and his Japanese passport, and explains that he is aware of the risks.
"Whatever happens, I am the one who is responsible," he says.
"I am asking you, Japanese people, do not place responsibility on the people of Syria. Please. I am sure I will come back alive though."