Japan Prime Minister Shinzo Abe on Sunday said he was “speechless” after an online video showed one of two Japanese hostages of Islamic State (IS) had been killed, and demanded the release of the other.

Mr. Abe said that the video was likely authentic and offered condolences to the family and friends of Haruna Yukawa, a 42-year-old adventurer taken hostage last year.

“I am left speechless,” he said, stressing he wants Goto, the other hostage, released unharmed.

“We strongly and totally criticise such acts.”

Yukawa’s father, Shoichi, on Sunday 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 said.

President Barack Obama condemned what he called “the brutal murder” of Yukawa, saying he stood by Japan to get Goto released.

The IS had threatened on Tuesday to behead the men within 72 hours unless it received a $200 million ransom.

Patrick Ventrell, a spokesman for the White House National Security Council, said U.S. intelligence officials were also working to confirm whether the video was authentic.

Mr. Abe said after a Cabinet meeting late on Saturday that the government of Japan will not succumb to terrorism and will continue to cooperate with the international community in the fight against terrorism.