Japan's prime minister said Sunday he was "speechless" after an online video purportedly showed an Islamic State militant killing one of the two Japanese hostages.

Shinzo Abe told Japanese broadcaster NHK the government is still reviewing the video, but it was likely authentic. Abe offered his condolences to the family and friends of 42-year-old Haruna Yukawa, who was taken hostage in Syria last year.

Abe did not comment about the message in the latest video that demanded a prisoner exchange for the other hostage, journalist Kenji Goto.

"I am left speechless," he said, stressing he wants Goto released unharmed. "We strongly and totally criticize such acts."

Yukawa's father, Shoichi, said he hoped "deep in his heart" that the news of his son's killing was not true.

"If I am ever reunited with him, I just want to give him a big hug," he told a small group of journalists invited into his house.

President Obama condemned the "brutal murder." He said in a statement that the United States stands by Japan and calling for Goto's release.

The U.S. National Security Council said it has seen the video and that the intelligence community is working to confirm its authenticity.

“The United States strongly condemns (Islamic State’s) actions, and we call for the immediate release of all the remaining hostages,” said agency spokesman Patrick Ventrell.

The CIA also has confirmed it is aware of the video and is reviewing it.

The video message claims one hostage has been killed and demands a prisoner exchange for the other.

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

Japanese government spokesman Yoshihide Suga said the audio was still being studied, but there was no reason to deny the authenticity of the video.

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.

The Associated Press contributed to this report