The news left Mr Goto's mother Junko Ishido distraught
Prime
Minister Abe said Japan would work with the international community to
bring those responsible for Mr Goto's apparent murder to justice
Japan
has reacted with anger and defiance to a video purporting to show the
beheading of Japanese hostage Kenji Goto by an Islamic State militant.
Prime
Minister Shinzo Abe said Japan "would not give in to terrorism" and
that he would expand his support to countries fighting IS.
IS has cited Japanese aid as a reason for the hostage taking.
The video comes less than a week after the apparent beheading of another Japanese man, Haruna Yukawa.
Mr
Goto, 47, a respected journalist known for his work covering the
suffering of civilians in war zones, went to Syria in October,
reportedly to try to secure Mr Yukawa's release.
The
video, which has all the hallmarks of previous IS propaganda videos,
has not been authenticated, but Japanese officials believe it is
genuine.
Mr
Abe called it a "heinous act", adding that Japan would work with the
international community to bring those responsible for Mr Goto's
apparent murder to justice.
There has been strong condemnation from the US and other allies in the fight against IS.
Mr Goto's mother Junko Ishido said she was speechless at his death, saying he had gone to Syria out of "kindness and courage".
"I was hoping Kenji would come back alive," his brother Junichi told Japanese broadcaster NHK TV.
US President Barack Obama said "the United States condemns the heinous murder of Japanese citizen and journalist Kenji Goto by the terrorist group ISIL (IS). Through his reporting, Mr Goto courageously sought to convey the plight of the Syrian people to the outside world".
British Prime Minister David Cameron said "I utterly condemn what appears to be the despicable and appalling murder of Kenji Goto. It is a further reminder that ISIL is the embodiment of evil, with no regard for human life".
French President Francois Hollande said he "resolutely condemned the brutal murder of Japanese citizen Kenji Goto by Daesh (IS). France stands in solidarity with Japan in this new ordeal".
Australian Prime Minister Tony Abbot said "if true, this will be a terrible shock for the people of Japan, an unspeakable horror for the family and this just demonstrates how important it is that all countries do what they reasonably can to disrupt and degrade this death cult which has ushered in a new dark age over parts of the Middle East".
There has been outrage in Japan over the killing
Japanese officials had been working with Jordan to secure the release of Mr Goto and a Jordanian pilot, Lt Moaz al-Kasasbeh, who was shot down over Syria in December.
However, earlier on Saturday they said negotiations had become deadlocked.
An IS video released on Tuesday had said Mr Goto had "only 24 hours left to live" and Lt Kasasbeh "even less".
They later gave a deadline of sunset on Thursday for a deal by which Mr Goto would be freed in return for Jordan releasing captured Iraqi militant Sajida al-Rishawi.
But the deal may have been complicated by Jordan's demand that Lt Kasasbeh also be released.
There is no further word on Jordanian pilot, Moaz al-Kasasbeh, also seized by IS
An adviser to the Japanese prime minister, Tomohiko Tanguchi, said his country had never made direct contact with IS throughout the negotiations.
The militants had initially demanded a $200m (£130m) ransom for the two Japanese hostages, the same as Mr Abe pledged in in non-military aid for countries fighting Islamic State.
The latest video gave no mention of Lt Kasasbeh's fate: his family said they were praying he was safe and urged the Jordanian government to do more to secure his release.
The BBC's Kevin Connolly in Amman says the danger for Jordan is that Islamic State may see Lt Kasasbeh as a hostage of particularly high value because of his role in the air campaign and may demand a very high political price for his freedom - if they can prove that he is still alive.
BBC