A visibly shaken Prime Minister Shinzo Abe said early Sunday that Japan was working to free two hostages held by the extremist Islamic State group, calling a new online video purporting to show that one had been killed "outrageous and unforgiveable."

The message claimed one of the Japanese hostages, Haruna Yukawa, had been killed and demanded a prisoner exchange for the other, Kenji Goto. But the post was deleted quickly Saturday, and militants on a website affiliated with the Islamic State group questioned its authenticity.

The Associated Press could not verify the contents of the message, which varied greatly from previous videos released by the Islamic State group, which now holds a third of both Syria and Iraq.

The Islamic State group had threatened on Tuesday to behead the men within 72 hours unless it received a $200 million ransom. Kyodo News agency reported that Saturday's video was emailed to Goto's wife.

Citing the release of the photo claiming to show hostage Yukawa had been killed, Abe said after a late-night Cabinet meeting: "Such an act of terrorism is outrageous and unforgivable. We feel strong indignation, and vehemently condemn the act."

Defense Minister Gen Nakatani said officials were trying to verify the video and the photo shown in it.

Patrick Ventrell, a spokesman for the White House National Security Council, said U.S. intelligence officials were also working to confirm whether it was authentic. "We stand in solidarity with Japan and are coordinating closely," he said, and called for the immediate release of people held by the Islamic State group.

Abe said the government of Japan will not succumb to terrorism and will continue to cooperate with the international community in the fight against terrorism. He said Japan is still taking every possible step to win the release of both hostages and will continue the effort.

Japan has been struggling to find a way to secure the release of Goto, a 47-year-old journalist, and Yukawa, a 42-year-old adventurer fascinated by war. Japanese diplomats left Syria as the civil war there escalated, compounding the difficulty of reaching the militants holding the hostages.

Abe spoke by phone with Jordanian King Abdullah II on Saturday, the state-run Petra news agency reported, without elaborating on what they discussed. He also called the two hostages' families.

Goto's mother, Junko Ishido, told Japanese public broadcaster NHK in a televised interview that in the purported message her son, "seemed to be taking seriously what may be happening to him as well."

"I'm petrified," Ishido said. "He has children. I'm praying he will return soon, and that's all I want."

But Ishido also was skeptical about the voice claiming to be Goto. "Kenji's English is very good. He should sound more fluent," she said.

One militant on the Islamic State-affiliated website warned that Saturday's new message was fake, while another said that the message was intended only to go to the Japanese journalist's family.

A third militant on the website noted that the video was not issued by al-Furqan, which is one of the media arms of the Islamic State group and has issued past videos involving hostages and beheadings. Saturday's message did not bear al-Furqan's logo.

The militants on the website post comments using pseudonyms, so their identities could not be independently confirmed by the AP. However, their confusion over the video matched that of Japanese officials and outside observers.

Japanese officials have not directly said whether they are considering paying any ransom. Japan has joined other major industrial nations in opposing ransom payments. U.S. and British officials said they advised against paying.

Foreign Minister Fumio Kishida said he planned to issue a safety warning to all Japanese citizens traveling outside the country through its embassies around the world.

The nightmarish situation had left him, "at a loss for words," Kishida said.

Nobuo Kimoto, a business adviser to Yukawa, told the Japanese broadcaster NHK: "I was hoping he would be released, or at least that his life would not be taken."

"I wish this was some kind of a mistake," he said.