TOKYO (REUTERS, AFP) - The father of a Japanese hostage purportedly killed by the Islamic State in Iraq and Syria (ISIS) said his mind went "totally blank" when he heard the news and he hoped the other Japanese hostage would be released soon.

A new video released on YouTube late on Saturday shows a still image of freelance journalist Kenji Goto, 47, holding an apparent photograph of 42-year-old Haruna Yukawa's slain body. There is an audio recording in which Goto speaks of the ISIS group's demand for a prisoner exchange to guarantee his release.

The father of Yukawa told the media on Sunday of the horror of realising the threats to kill his son had been carried out.

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

"I went totally blank, I was only sorry... I had no words to say," he said. "In my mind I wish very much that this wasn't true.

"I am filled with disappointment that it has finally come to this."

Shoichi Yukawa repeatedly apologised to Goto, who was understood to have travelled to Syria to try to free his son.

"We are very sorry for causing trouble" to the public as well as to Goto, the father said. "We are deeply grateful that the government and others concerned have made their utmost efforts."

He said his son had felt as if Goto was his "big brother". "My son told me all the time that he is a sincere, very courageous and gentle man.

"I feel it very painful that Mr Goto worried about Haruna, went there and risked his own life and then was kidnapped and threatened this way," he said. "I hope he can be released as soon as possible, and return to Japan to continue his activities."

Goto's mother, Junko Ishido, told NHK: "Of course, first of all, I wish it weren't true, that it's some mistake. I'm a mother so it's unbearable. What I want to tell Islamic State is that Kenji's ideal is world peace."

On Sunday, more than 100 people, some wiping away tears, congregated at Denenchofu Protestant evangelical church in an upscale neighbourhood of Tokyo, where Goto was baptised in 1997 and where he prayed just days after Yukawa was captured in August in Syria. Three policemen stood guard outside the church.

"Please have Kenji Goto and Haruna Yukawa in your thoughts as we go through today's prayers," Pastor Shun Takatsu said.

In the latest video, Goto is heard saying "they no longer want money. So, you don't need to worry about funding terrorists."

"They are just demanding the release of their imprisoned sister Sajida al-Rishawi. It is simple. You give them Sajida and I will be released."

Sajida is an Iraqi woman who was arrested after she tried to detonate an explosive belt at a wedding party at the Radisson Hotel in Amman in 2005.

A deal is unlikely since it would involve a public concession to the militant group and strain relations with the United States, a key economic and defence ally for Japan and Jordan.

Yukawa, a self-styled military contractor, was captured in August outside the Syrian city of Aleppo. Goto, a war correspondent with experience in Middle East hot spots, reportedly went to Syria in late October to try to help Yukawa.

"He left a very young baby and left his family and I asked his wife why he made this decision and she said he had to do everything in his power to save his friend and acquaintance and that it was very important to him," Goto's mother said on Friday.

In the first online video released on Tuesday, a black-clad figure holding a knife stood between Goto and Yukawa, threatening to kill them if Tokyo did not pay ISIS US$200 million (S$270 million) within 72 hours.

The Japanese government considered the deadline to be 2.50pm local time on Friday (1.50pm Friday Singapore time).