"We will be vigilant and we will be relentless," Obama said.
Calling for a global response to the group that now controls territory in both Iraq and Syria, Obama condemned the group's execution of journalist James Foley, whose death he said had left the nation heartbroken. In forceful remarks, Obama accused the Islamic State of torturing, raping and murdering thousands of people in "cowardly acts of violence."
"ISIL speaks for no religion," Obama said, using an alternative name for the Islamic State. "Their victims are overwhelmingly Muslim, and no faith teaches people to massacre innocents. No just god would stand for what they did yesterday and what they do every single day."
Obama's remarks affirmed that the U.S. would not change its military posture in Iraq in response to Foley's killing.
Since the video was released Tuesday, the U.S. military has pressed ahead by conducting nearly a dozen airstrikes on Islamic State targets in Iraq. And on Wednesday, U.S. officials said military planners were considering the possibility of sending a small number of additional troops to Iraq, mainly to provide additional security around Baghdad.
Obama said he'd told Foley's family in a phone call Wednesday that the United States joins them in honoring all that Foley did, praising the journalist for his work telling the story of the crisis in Syria, where Foley was captured in 2012. "Jim Foley's life stands in stark contrast to his killers," Obama said. He spoke from Martha's Vineyard in Massachusetts, where his family is vacationing.
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.
"Today, the American people will all say a prayer for those who loved Jim," Obama said. "All of us feel the ache of his absence. All of us mourn his loss."
The slain journalist's parents, Diane and John Foley, spoke to reporters outside their Rochester home, in an appearance where wrenching grief over their son's death mingled with laughter over his life.
Diane Foley said her son was courageous to the end and called his death "just evil."
The beheading marks the first time the Islamic State 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-Qaida 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 the Islamic State group's media unit and begins with scenes of Obama explaining his decision to order airstrikes.
It then cuts to Foley, kneeling in the desert. After the captive speaks, a masked militant is shown apparently beginning to cut at Foley's neck. The video fades to black before the beheading is completed; the next shot shows him lying dead. 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.
The militant in the video has not been identified, but he spoke with a British accent, and British Prime Minister David Cameron said that "from what we have seen it looks increasingly likely that is a British citizen."
Obama did not specifically mention Steven Sotloff, another kidnapped American journalist that Islamic State says could be killed next. But he offered prayers on behalf of the American people for "those other Americans who are separated from their families."
A man identified as Sotloff appears at the end of the Foley video in a similar orange jumpsuit. Sotloff was kidnapped near the Syrian-Turkish border in August 2013; he had freelanced for Time, the National Interest and MediaLine.
Tuesday's airstrikes by American fighter jets and drones centered on targets around the Mosul Dam and were designed to help Iraqi and Kurdish forces create a buffer zone at the key facility, according to a U.S. official. The official was not authorized to discuss the ongoing operations publicly so spoke on condition of anonymity.
Since Aug. 8, there have been nearly 90 U.S. airstrikes in Iraq on Islamic State 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.
The Islamic State militant group is so ruthless in its attacks against all people they consider heretics or infidels that it has been disowned by al-Qaida'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.
Senators from both parties condemned the killing, and some Republicans questioned Obama's resolve in confronting the Islamic State.
"The United States must not cower to these terrorists' ruthless demands by remaining on the sidelines," said Rep. Mike Pompeo, R-Kan., a House Intelligence Committee member and former Army officer.
"ISIL has declared war on the United States, on the American people and on freedom loving people everywhere," added Sen. Marco Rubio, R-Fla. He said Obama has been "unwilling to do what is necessary to confront ISIL."
The New York-based Committee to Protect Journalists says that more than 80 journalists have been abducted in Syria, and estimates that around 20 are currently missing there. It has not released their nationalities. In its annual report in November, the committee 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.