President Barack Obama condemned on Saturday the "heinous murder" of a Japanese hostage after the Islamic State 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," 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 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."
He was the second Japanese hostage beheaded by IS, 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 jihadists had asked Tokyo to pay a $200 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," 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."
The apparent execution came after Japan said negotiations to win Goto's release in a prisoner exchange had stalled.
The IS group had vowed to kill Goto and Jordanian pilot Maaz al-Kassasbeh by sunset Thursday unless Amman handed over an Iraqi female jihadist.
White House National Security Council spokeswoman 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," Obama vowed.
His defence secretary Chuck Hagel said Goto's execution, along with that of 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," Hagel added.
Secretary of State John Kerry also spoke out, condemning what he described as Goto's "barbaric killing."