Prime Minister Shinzo Abe said Sunday that the “credibility” of the online video apparently posted by the Islamic State group Saturday is “high,” suggesting Tokyo believes Japanese hostage Haruna Yukawa was actually killed by the extremists as claimed in the clip, which includes a shocking still image of what appeared to be his corpse.

“To our regret, we have no choice but say to the credibility (of the video) is high,” Abe said during a live interview with NHK. The prime minister had appeared on the program as earlier scheduled.

“Such an act of terrorism is outrageous and impermissible,” Abe said. “I feel strong indignation. I resolutely condemn this.”

The Islamic State group had threatened to kill journalist Kenji Goto and Haruna Yukawa, a self-styled private security contractor, in a separate video posted on the Internet on Tuesday, demanding Japan pay $200 million within 72 hours.

The video released Saturday showed a still image of Goto holding a photo of what appeared to be Yukawa’s beheaded body.

Some experts and media outlets have questioned the authenticity of the video clip, which was posted on the Internet around 11 p.m. Saturday.

The clip did not bear the logo of al-Furqan, a media arm of the Islamic State group that has issued past videos involving hostages and beheadings.

Junko Ishido, Goto’s mother, told reporters in Tokyo that she does not believe the voice of the man on the video is that of her son.

The government, however, is taking the claims made in the video very seriously.

“We will make further efforts to seek the release (of Goto). And we will not give into terrorism,” Abe told NHK.

“We won’t change this stance of ours,” he added.

The man’s voice on the video identified himself as Goto and, speaking in English, claimed that the Islamic State group had executed Yukawa because Abe had failed to pay the ransom by the deadline.

In the video, the voice attributed to Goto also said the Islamic State group was abandoning its ransom demand and instead urged the release of Sajida Mubarak Atrous al-Rishawi in exchange of Goto.

Al-Rishawi, reportedly an Iraqi, was the wife of Ali Hussein Ali al-Shamari, who, together with three other suicide bombers killed 57 people in three hotels in the Jordanian capital of Amman in 2005.

Al-Rishawi is reportedly imprisoned in Jordan. Questioned if Tokyo would ask Amman to release her in exchange of Goto, Abe declined to comment.

“We’d like to refrain from discussing how we will deal with (it) because the situation is changing right now,” Abe said.

“At any rate, we will put our top priority on (saving Goto’s life) and closely cooperate with Jordan to cope with the situation,” he added.

“We should stop this tide of extremism,” Abe said.

The prime minister also said the stability of the Middle East is “critically important” for Japan, which is heavily dependent on oil exports from the region.

Abe vowed that Japan would continue giving food, medical and other humanitarian aid to evacuees who have fled areas controlled by the Islamic State group.

“This is aid that evacuees keenly need,” the prime minister said. “We shouldn’t changed our policy” of providing such assistance.

Later the day, Chief Cabinet Secretary Yoshihide Suga faced reporters at an 11 a.m. news conference. Asked if Tokyo had had any direct communications with the Islamic State group other than the release of videos, Suga said: “No, we haven’t.”

Meanwhile, a high-ranking Japanese official said that the government had analyzed in detail the image of what appeared to be Yukawa’s corpse, including its facial features.

“We have found nothing to deny” the group’s claim that it has executed Yukawa, the official said.