Prime Minister Shinzo Abe on Sunday branded the murder of a Japanese hostage by Islamic State militants as "outrageous and unforgivable" and demanded the immediate release of a second captive, amid growing global revulsion.

The apparent beheading of self-employed security contractor Haruna Yukawa was announced in a video posted online by militants, and appeared to mark a grave turn of events in a crisis that has gripped Japan for nearly a week.

"Such an act of terrorism is outrageous and unforgivable," Abe told broadcaster NHK.

"I condemn it strongly and resolutely," he said, calling for the immediate freeing of Yukawa's fellow captive, freelance journalist Kenji Goto.

In a city outside Tokyo, Shoichi Yukawa told of the horror he had felt when he learnt that threats to kill his son had been carried out.

"I thought 'Ah, this finally happened' and was filled with regret," he said.

"I went totally blank, I was only sorry... I had no words," he said. "In mymind Iwish very much that this wasn't true."

US President Barack Obama led the worldwide condemnation of what he called the Islamic State group's "brutal murder" of Yukawa.

British Prime Minister David Cameron decried the movement's "murderous barbarity", and French President Francois Hollande labelled it a "barbaric assassination".

Australia's Tony Abbot called it "an absolute atrocity" carried out by a "death cult".



$200 million ransom

Japan was continuing toanalysethe images released overnight to confirm the authenticity of the video, said Abe, but he acknowledged it appeared credible.

The recording, which lasts nearly three minutes, shows a still image of Goto holding what appears to be a photograph of Yukawa's slain body.

It was posted with an audio recording in which a man claiming to be Goto blames Abe for his fellow captive's death because he failed to pay the $200 million ransom thejihadistsdemanded.

The voice also reveals a new demand for the release of Sajida al-Rishawi, an Iraqi woman sentenced to death in Jordan for her part in multiple bombings in Amman in 2005 that killed 60 people.

The man, who speaks accented English, said the militants are no longer demanding money to save his life, but want "their sister" to be freed.

"It is simple. You give them Sajida and I will be released," the voice says. "At the moment, it actually looks possible and our governmentareindeed a stone throw away."

Some commentators have noted that the voice does not resemble that heard on videos featuring Goto. However, one analyst interviewed by Japanese media was reported to have said it was "more than 99 percent certain" the voice was his.

Japan dispatched a senior minister to Jordan earlier this week. Abe declined to comment on whether he would ask Amman to release Sajida.



'Very tense'

The appearance of the video was met with initialscepticism, partly because it was not posted on an official IS channel and does not bear their usual black and white flag. The killing of Yukawa is also not shown.

Goto'smother Junko Ishido told reporters Sunday she was left with little hope after seeing the picture of her son, in which he "looked very tense".

"Japan never abandons its people," she said. "I believe the government is united and doing its best."

The Islamic State group, which rules large swathes 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.

Japanese officials have repeatedly said they are trying to make contact with the militants. One adviser to Abe 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 had previously said Abe may try to use his close relationship with Turkey's President Recep Tayyip Erdogan to rescue the hostages.

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.

Abe was defiant Sunday.

"We will never give in to terrorism, and we will actively contribute to the peace and stability of the world together with the international community. We are not wavering at all on this policy," he said.