Japan and other nations condemned with outrage and horror on Sunday the beheading purportedly by the Islamic State group of Kenji Goto, a journalist who sought through his coverage of Syria to convey the plight of refugees, children and other victims of war. 

The failure to save Goto raised fears for the life of a Jordanian fighter pilot also held hostage by the extremists. Unlike earlier messages, an online video purporting to show an Islamic State group militant beheading Goto, circulated via social media late Saturday by militant sympathizers, did not mention the pilot.

Goto's slaying shocked this country, which up to now had not become directly embroiled in the fight against the militants. 

"I feel indignation over this immoral and heinous act of terrorism," Prime Minister Shinzo Abe told reporters after convening an emergency Cabinet meeting.

"When I think of the grief of his family, I am left speechless," he said. "The government has been doing its utmost in responding to win his release, and we are filled with deep regret."

In light of threats from the Islamic State group, the government ordered heightened security at airports and at Japanese facilities overseas, such as embassies and schools, government spokesman Yoshihide Suga said.

He said it would be "inappropriate" to comment on the status of the Jordanian pilot, Muath al-Kaseasbeh. He was captured in December when his F-16 crashed near the de facto capital of the Islamic State group, which controls about a third of both Syria and neighboring Iraq in a self-declared caliphate.

Jordan's government spokesman, Mohammed al-Momani, also declined comment. Earlier this week, Jordan offered to free an al-Qaida prisoner for the pilot, but demanded and said it never got proof he was still alive.

Goto, 47, was a freelance journalist and father who braved hardship and peril to convey the suffering caused by conflict and poverty. 

"Kenji has died, and my heart is broken. Facing such a tragic death, I'm just speechless," Goto's mother Junko Ishido told reporters.  

"I was hoping Kenji might be able to come home," said Goto's brother, Junichi Goto, in a separate interview. "I was hoping he would return and thank everyone for his rescue, but that's impossible, and I'm bitterly disappointed."

Japanese expressed shock and horror over Goto's killing.

Yukawa's father, Shoichi, said Goto was trying to rescue his son"only to suffer the worst possible outcome.''

"I just have no words. It's utterly heartbreaking," he said. "People killing other people -- it's so deplorable. How can this be happening?" 

Abe vowed not to give in to terrorism and said Japan will continue to provide humanitarian aid to countries fighting the Islamic State extremists.  

The defense minister, Gen Nakatani, said that the police agency had deemed the video of Goto's killing "highly likely to be authentic."

According to his friends and family, Goto traveled to Syria in late October to try to save another hostage, Haruna Yukawa, who was captured by the Islamic State group in August and shown as purportedly killed in an earlier video. 

The White House released a statement in which President Barack Obama also condemned "the heinous murder" and praised Goto's reporting, saying he "courageously sought to convey the plight of the Syrian people to the outside world." 

The White House said that while it isn't confirming the authenticity of the video itself, it has confirmed that Goto has been slain. 

Saturday's video, highlighted by militant sympathizers on social media sites, bore the symbol of the Islamic State group's al-Furqan media arm.  

Though it could not be immediately independently verified by The Associated Press, it conformed to other beheading videos released by the extremists, who now control about a third of both Syria and neighboring Iraq in a self-declared caliphate.

In Jordan late Saturday night, relatives and supporters of the pilot held a candlelit vigil inside a family home in Karak, al-Kaseasbeh's hometown in southern Jordan.  

We "decided to hold this protest to remind the Jordanian government of the issue of the imprisoned pilot Muath al-Kaseasbeh," said the pilot's brother Jawdat al-Kaseasbeh, holding picture of Muath with a caption: "We are all Muath." 

Al-Kaseasbeh's uncle, Yassin Rawashda, said the family just wants to be kept informed.

"We want to know how the negotiations are going ... in a positive direction or not. And we want the family to be (involved) in the course of negotiations," he said. 

In a purported online message earlier this week, the militants threatened to kill the pilot if the al-Qaida prisoner, 44-year-old Sajijda al-Rishawi, wasn't released by sunset on Thursday. That deadline passed, leaving the families of the pilot and the journalist waiting in agony. 

Jordan and Japan had reportedly conducted indirect negotiations with the militants through Iraqi tribal leaders, but late on Friday Japan's deputy foreign minister reported a deadlock in those efforts.

The hostage drama began last week when the militants threatened to kill Goto and Yukawa in 72 hours unless Japan paid $200 million.

Later, the militants' demand shifted to seeking the release of al-Rishawi, who is facing death by hanging in Jordan for her role in triple hotel bombings in Amman in 2005. Sixty people were killed in those attacks, the worst terror attack in Jordan's history.

Al-Rishawi has close family ties to the Iraq branch of al-Qaida, a precursor of the Islamic State group. (AP)