WASHINGTON (AFP) - United States President Barack Obama joined some other Western leaders in condemning on Saturday the "heinous murder" of a Japanese hostage, after the Islamic State in Iraq and Syria (ISIS) group released a video purportedly showing his beheading.

"The United States condemns the heinous murder of Japanese citizen and journalist Kenji Goto by the terrorist group ISIL," Mr Obama said in a statement, using an acronym by which the IS group is known. "Through his reporting, Mr Goto courageously sought to convey the plight of the Syrian people to the outside world."

The claim was made in a video released online that included no mention of a Jordanian pilot also being held by ISIS, whom the 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 ISIS militant who has featured in the group's previous execution videos.

Mr Goto was the second Japanese hostage beheaded by ISIS, after the group claimed responsibility last week for the killing of self-described contractor Haruna Yukawa, after the expiration of a 72-hour deadline during which the militants had asked Tokyo to pay a US$200 million (S$266 million) ransom.

"Our thoughts are with Mr Goto's family and loved ones, and we stand today in solidarity with Prime Minister Abe and the Japanese people in denouncing this barbaric act," Mr Obama said.

"We applaud Japan's steadfast commitment to advancing peace and prosperity in the Middle East and globally, including its generous assistance for innocent people affected by the conflicts in the region."

White House National Security Council spokesman Bernadette Meehan said earlier that the United States was working to confirm the video's authenticity.

"Standing together with a broad coalition of allies and partners, the United States will continue taking decisive action to degrade and ultimately destroy ISIL," Mr Obama vowed.

US Defence Secretary Chuck Hagel said Mr Goto's execution, along with that of Mr Yukawa, "reminds the world of the threat we confront in the Islamic State of Iraq and the Levant, while it reinforces our global coalition's commitment to degrade and ultimately destroy ISIL."

"The United States and its military will continue to stand alongside our Japanese allies, and we send our thoughts and prayers to Mr Goto's loved ones and all the people of Japan," Mr Hagel added.

Secretary of State John Kerry also spoke out, condemning what he described as Mr Goto's "barbaric killing".

French President Francois Hollande Sunday "resolutely" condemned the apparent execution.

Mr Hollande said he "resolutely condemned the brutal murder of Japanese citizen Kenji Goto by Daesh", using an alternative acronym for the group, according to a statement from his office.

"France stands in solidarity with Japan in this new ordeal," Mr Hollande said, adding that France and Japan would "continue to work together for peace in the Middle East and to eliminate terrorist groups".

British Prime Minister David Cameron on Saturday condemned as "appalling" the purported killing.

"I utterly condemn what appears to be the despicable and appalling murder of Kenji Goto," Mr Cameron said in a statement.

"It is a further reminder that ISIL (ISIS) is the embodiment of evil, with no regard for human life."

Mr Cameron added that Japan was "right not to bow to terrorism" following the killing of Mr Goto and another Japanese citizen, Mr Haruna Yukawa.

"The way we will defeat ISIL is not by giving in to these terrorists but by confronting them and their poisonous ideology," the British leader said.

Britain had been working with Japan to share knowledge of handling hostage situations, government officials said last week.

Two British citizens, Mr David Haines and Mr Alan Henning, were killed by ISIS last year.

A spokesman for UN chief Ban Ki Moon also condemned the "barbaric murder", and said the death "underscores the violence that so many have been subjected to in Iraq and Syria".

The apparent execution came after Japan said negotiations to win Mr Goto's release in a prisoner exchange had stalled.

ISIS had vowed to kill Mr Goto and Jordanian pilot Maaz al-Kassasbeh by sunset on Thursday unless Amman handed over an Iraqi female militant.

Japanese Prime Minister Shinzo Abe on Sunday condemned the apparent killing, vowing to work with other nations to bring those responsible to justice.

"I am extremely angry about these heinous and despicable terrorist acts. We will never forgive terrorists," the Premier, who appeared on the verge of tears, told reporters.

"I feel strong indignation at this inhumane and contemptible act of terrorism," a grim-faced Mr Abe said in brief remarks to reporters.

"I will never forgive these terrorists.

"Japan will work with the international community to bring those responsible for this crime to justice," he added, reiterating that Japan would not give in to terrorism.