How will the U.S. respond to ISIS after the a...

WASHINGTON -- Militants with an Islamic extremist group who beheaded American journalist James Foley are threatening to kill another hostage.

In a horrifying act of revenge for U.S. airstrikes in northern Iraq, a grisly video released Tuesday shows militants from the Islamic State of Iraq and Syria, or ISIS, beheading Foley.

A masked Islamic State of Iraq and Syria militant speaks next to a man purported to be U.S. journalist Steven Sotloff at an unknown location in this still image from an undated video posted on a social media website. Reuters/social media website via Reuters TV

At the end of the video, a militant shows a second man, who was reported to be another American journalist, Steven Sotloff, and warns that he could be the next captive killed.

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On Wednesday, U.S. National Security Council spokeswoman Caitlin Hayden said in a statement that the intelligence community determined the video to be authentic.

Sotloff was kidnapped near the Syrian-Turkish border in August 2013; he had freelanced for Time, the National Interest and MediaLine.

The White House must now weigh the risks of adopting an aggressive policy to destroy ISIS, also known as the Islamic State of Iraq and the Levant, or ISIL, against resisting any action that could result in the death of another American.

It will also confront the potentially necessary step of pursuing ISIS in Syria, where President Obama has resisted launching airstrikes or deploying significant American firepower.

In a nationally broadcast statement at midday Wednesday, a visibly angry Mr. Obama said the world was "appalled" by the killing of Foley and compared the militants to a cancer that must be extracted from the region.

Foley's family confirmed his death in a statement posted on a Facebook page that was created to rally support for his release, saying they "have never been prouder of him."

"He gave his life trying to expose the world to the suffering of the Syrian people," said the statement, which was attributed to Foley's mother, Diane Foley. She implored the militants to spare the lives of other hostages. "Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world."

Foley, 40, from Rochester, New Hampshire, went missing in northern Syria in November 2012 while freelancing for Agence France-Presse and the Boston-based media company GlobalPost. The car he was riding in was stopped by four militants in a contested battle zone that both Sunni rebel fighters and government forces were trying to control. He had not been heard from since.

The beheading marks the first time ISIS has killed an American citizen since the Syrian conflict broke out in March 2011, upping the stakes in an increasingly chaotic and multilayered war. The killing is likely to complicate U.S. involvement in Iraq and the Obama administration's efforts to contain the group as it expands in both Iraq and Syria.

The group is the heir apparent of the militancy known as al Qaeda in Iraq, which beheaded many of its victims, including American businessman Nicholas Berg in 2004.

The video released on websites Tuesday appears to show the increasing sophistication of ISIS' media unit and begins with scenes of Mr. Obama explaining his decision to order airstrikes.

It then cuts to a balding man in an orange jumpsuit kneeling in the desert, next to a black-clad militant with a knife to his throat. Foley's name appears in both English and Arabic graphics on screen. After the captive speaks, the masked man is shown apparently beginning to cut at his neck; the video fades to black before the beheading is completed. The next shot appears to show the captive lying dead on the ground, his head on his body. The video appears to have been shot in an arid area; there is no vegetation to be seen and the horizon is in the distance where the sand meets the gray-blue sky.

One U.S. official told The Associated Press that the video appeared to be authentic, and two other U.S. officials told the AP the victim was Foley. All three officials spoke on condition of anonymity because they were not authorized to discuss the killing by name.

Several senior U.S. officials with direct knowledge of the situation told the AP ISIS very recently threatened to kill Foley to avenge the crushing airstrikes over the past two weeks against militants advancing on Mount Sinjar, the Mosul dam and the Kurdish capital of Erbil.

Both areas are in northern Iraq, which has become a key front for the ISIS as its fighters travel to and from Syria.

Since Aug. 8, the U.S. military has struck at least 70 ISIS targets - including security checkpoints, vehicles and weapons caches. It's not clear how many militants have been killed in the strikes, although it's likely that some were.

ISIS is so ruthless in its attacks against all people they consider heretics or infidels that it has been disowned by al Qaeda's leaders. In seeking to impose its harsh interpretation of Islamic law in the lands it is trying to control, the extremists have slain soldiers and civilians alike in horrifying ways - including mounting the decapitated heads of some of its victims on spikes.

The New York-based Committee to Protect Journalists estimated Tuesday that about 20 journalists are missing in Syria, and has not released their nationalities. In its annual report in November, the committee concluded that the missing journalists were either being held and threatened with death by extremists, or taken captive by gangs seeking ransom. The group's report described the widespread seizure of journalists as unprecedented and largely unreported by news organizations in the hope that keeping the kidnappings out of public view may help in the captives' release.