The Islamic State group said in a video on Saturday it had beheaded a second Japanese hostage, drawing outrage from Tokyo and condemnation from Washington.
The claim was made in a video released online that included no mention of a Jordanian pilot also being held by IS, whom the jihadist group has also threatened to kill.
Japanese journalist Kenji Goto is seen kneeling next to a standing masked man who speaks with a British accent and blames the Japanese government for his "slaughter".
The man, dressed head-to-toe in black with his face covered, appears to be the same IS militant who has featured in the group's previous execution videos.
Goto is dressed in an orange outfit similar to those worn by prisoners at the US-run prison in Guantanamo Bay, also in keeping with hostages in previous IS videos.
The executioner addresses Japanese Prime Minister Shinzo Abe, saying the murder of Goto would mark the beginning of "the nightmare for Japan".
Goto's killing, he said, was the result of "reckless" decisions by the Japanese government.
The brief video ends with the image of a body dressed in orange with a decapitated head on top of it.
In Tokyo, a visibly upset Abe vowed to "never forgive terrorists" after news of the video broke early Sunday morning in Japan.
"I am extremely angry about these heinous and despicable terrorist acts. We will never forgive terrorists," the premier told reporters, adding that he was "at a loss for words about the pain that (Goto's) family must feel".
"We will cooperate with the international community to make them atone for their crimes."
US President Barack Obama condemned the "heinous murder" of Goto.
"Through his reporting, Mr Goto courageously sought to convey the plight of the Syrian people to the outside world," Obama said.
National Security Council spokesperson Bernadette Meehan said earlier the United States was working to confirm the video's authenticity.
British Prime Minister David Cameron denounced "what appears to be the despicable and appalling murder of Kenji Goto", adding: "It is a further reminder that ISIL (IS) is the embodiment of evil, with no regard for human life."
French President Francois Hollande also condemned the "brutal murder".
The apparent execution came after Japan said negotiations to win Goto's release in a prisoner exchange had stalled.
"It has become deadlocked," deputy foreign minister Yasuhide Nakayama, who is leading Tokyo's emergency response in Jordan, told reporters in Amman late Friday, Japanese media reported.
IS had vowed to kill Goto and Jordanian pilot Maaz al-Kassasbeh by sunset Thursday unless Amman handed over an Iraqi female jihadist.
On Saturday morning Abe had renewed orders for officials to maintain close cooperation with Jordan in a bid to secure Goto's release.
"The government has been working with the utmost efforts on the issue -- I deeply regret that this is the result," Abe told reporters.
"Japan will never yield to terrorism...(and) is firmly resolved to fulfil its responsibility in the international community's fight against terrorism." Jordan has demanded evidence that the pilot, who crashed in Syria on December 24, was still alive before freeing would-be suicide bomber Sajida al-Rishawi, who is on death row
The latest video made no mention of Kassasbeh's fate.
Last week IS claimed responsibility for the beheading of another Japanese man it had been holding, self-described contractor Haruna Yukawa, after the expiration of a 72-hour deadline during which the jihadists had asked Tokyo to pay a $200 million (175 million euro) ransom.
'Proof of life'
Jordan has offered to free Rishawi, who was convicted for her part in triple-hotel bombings in Amman in 2005 that killed 60 people, if IS releases the pilot.
The government has been under heavy pressure at home and from Japan -- a major aid donor -- to save Kassasbeh as well as Goto.
On Thursday, government spokesperson Mohammad al-Momani said Rishawi was still in Jordan and would only be released if IS gave it "proof of life".
IS had set the Thursday sunset deadline for Rishawi to be released at the Turkish border in return for Goto but there was no news of a swap by nightfall.
Friday morning Jordan's military said it was still awaiting proof that Kassasbeh was safe.
The pilot's father Safi Kassasbeh begged Amman to save his son's life "at any price".
"We believe in God and we will accept whatever he has in store for us," said Safi Kassasbeh
Goto's wife Rinko also broke her silence this week also to plead for her husband's return.
"My husband is a good and honest man who went to Syria to show the plight of those who suffer," she said.
"I beg the Jordanian and Japanese governments to understand that the fates of both men are in their hands."
IS has imposed a brutal version of Islamic law in territory it controls in Syria and Iraq and has executed since August two US journalists, an American aid worker and two British aid workers.