U.S. President Barack Obama has said the whole world is appalled by the beheading of American journalist James Foley by jihadist militants of the Islamic State (IS) group.
IS has said Foley's beheading was in revenge for U.S. air strikes against the insurgents in Iraq.
Speaking on August 20 in a first reaction to Foley's beheading -- shown on a video titled "A Message To America" and released by IS on August 19 -- Obama said, "No just god would stand for what they did yesterday."
The U.S. president called IS "a cancer" and said "their ideology is bankrupt."
Obama, speaking from Martha's Vineyard in Massachusetts, where he's vacationing with his family, said the United States will continue to confront the extremists despite threats to Americans, and "will do what it must do" to protect its people.
Islamic State "has no place in the 21st century," he said.
Separately, U.S. Secretary of State John Kerry said in a statement that the United States "will never back down in the face of such evil."
Islamic State "and the wickedness it represents must be destroyed, and those responsible for this heinous, vicious atrocity will be held accountable," he added.
The victim was identified by the militants as Foley, a freelancer for the French news agency AFP and the U.S. media company GlobalPost, who was kidnapped in Syria in November 2012.
Meanwhile, the Pentagon says the U.S. military recently attempted a rescue operation to free U.S. hostages held by Islamic State militants in Syria but failed to find them.
In a statement, the Pentagon said the operation had involved "air and ground components."
President Barack Obama authorized the mission "earlier this summer," said a statement by Lisa Monaco, Obama's top counterterrorism aide.
Officials would not say exactly when the operation took place but said it was not in the last couple of weeks.
Meanwhile, Foley's mother, Diane, wrote on Facebook: "We have never been prouder of our son Jim. He gave his life trying to expose the world to the suffering of the Syrian people."
British Prime Minister David Cameron interrupted his holiday to return to London to lead the hunt to identify the man shown killing Foley, who spoke on the video with a British accent.
Cameron said on Twitter that "...the murder of James Foley is shocking and depraved. I will today chair meetings on the situation in Iraq/Syria."
British Foreign Secretary Philip Hammond said the man in the video with Foley "appears to have been a British person."
French Foreign Minister Laurent Fabius said that the "disgusting assassination would show the true face of this 'caliphate of barbarism,'" while President Francois Hollande called for an international conference to discuss how to tackle IS.
Hollande told the daily "Le Monde" that "we have to come up with a global strategy to fight this group," which he said "threatens countries like Iraq, Syria, and Lebanon."
The footage also shows a second captive alive, identified by the militants as U.S. reporter Steven Sotloff, who disappeared in Syria in the summer of 2013. The militant in the video warns that Sotloff could be killed next.
Fighting Continues In Iraq
Meanwhile, Foreign Minister Frank-Walter Steinmeier said Germany was ready to send weapons to support Iraqi Kurds in their battle against the "barbaric" IS.
And Italian Defense Minister Roberta Pinotti said on August 20 that her government was ready to arm Iraqi forces fighting Islamist militants with Kalashnikov rifles.
In a related development, U.S. officials said U.S. aircraft have launched 14 air strikes against IS fighters near Iraq's largest dam in the past 24 hours.
The strikes came in the hours after militants released the video of Foley's beheading.
The latest air strikes were in the area of the Mosul dam and were aimed at helping Iraqi and Kurdish forces create a buffer zone at the strategic facility.
News agencies also quoted unnamed U.S. officials as saying military planners weighed the possibility of sending 300 additional U.S. troops to Baghdad.
Currently there are about 748 U.S. soldiers in Iraq, in addition to the approximately 100 troops that have routinely been assigned to the Office of Security Cooperation in Baghdad.
Islamic State, a militant group that seeks hard-line rule under an Islamic caliphate, has seized large swaths of territory in both Iraq and Syria.
The militants have been accused of massacring hundreds of people in areas under their control.
They have persecuted non-Muslims such as Yazidis and Christians, as well as Shi'ite Muslims.
The group's recent advances in northern Iraq have prompted the United States to launch air strikes in the Middle Eastern nation for the first time since its withdrawal in 2011.