“I thought ‘Ah, this has finally happened’ and was filled with regret,” Shoichi Yukawa said just hours after a video appeared online claiming that his son Haruna Yukawa, 42, had been executed.

“I went totally blank, I was only sorry ... I had no words to say,” the 74-year-old said.

The announcement of Haruna Yukawa’s death came days after the Islamic State group published a video in which it threatened to kill him and freelance journalist Kenji Goto unless Japan paid a $US200 million ($252 million) ransom within 72 hours.

That deadline passed on Friday, with Tokyo saying it was still making frantic efforts to contact the jihadists.

JAPANESE HOSTAGES: ISIS video purports to show hostage beheading

DEADLINE PASSES: IS militants claim to have executed Japanese hostages

Heartache ... Shoichi Yukawa (right, in blue), father of Haruna Yukawa, said his mind went blank when he found out his son was murdered. Picture: AFP/Yasuhiro Sugimoto Source: AFP

In the accompanying audio recording, a man claiming to be Goto blames Japanese Prime Minister Shinzo Abe for Yukawa’s killing, and demands the release of an Iraqi woman sentenced to death in Jordan for her part in multiple bombings.

Shoichi Yukawa on Sunday repeatedly apologised to Goto, who had travelled to Syria to try to free his son.

Japanese hostage Kenji Goto. Picture: AFP/SITE Intelligence Group Source: AFP

Japanese hostage Haruna Yukawa. Picture: AP Photo/Eugene Hoshiko Source: AP

“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.”

The father said his son had felt as if Goto, 47, 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 had been hoping something like this wouldn’t happen, but it finally did. If I could see him again, I would hug him with all my strength.”

Heartbroken ... Junko Ishido, mother of Japanese journalist Kenji Goto who was taken hostage by the Islamic State group, reacts during a press conference in Tokyo. Picture: AP Photo/Koji Sasahara Source: AP

Japan PM: ‘Outrageous and unforgivable act of terrorism’

Meanwhile, Prime Minister Shinzo Abe has 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.

“Such an act of terrorism is outrageous and unforgivable,” Abe told broadcaster NHK on Sunday.

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

Outraged ... Japanese Prime Minister Shinzo branded the murder of a Japanese hostage by Islamic State militants as ‘outrageous and unforgivable’. Picture: AFP/Yoshikazu Tsuno Source: Supplied

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

Obama, who arrived in New Delhi on Sunday for a three-day visit, telephoned Abe from the Indian capital “to offer condolences for the murder ... of Japanese citizen Haruna Yukawa and to convey solidarity with the Japanese people”, said a White House statement.

Protests ... anti-Prime Minister Shinzo Abe protesters rally with signs and a banner reading: ‘Prime Minister Abe, save the life of Kenji Goto!’ outside the leader’s official residence in Tokyo. Picture: AP Photo/Koji Sasahara Source: AP

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

Australia’s Prime Minister Tony Abbott called it “an absolute atrocity” carried out by a “death cult”.

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