Obama vows to 'degrade and destroy' ISIS as beheading video deemed authentic

President Barack Obama says the United States won't be intimidated by Islamic State militants after the beheading of a second American journalist and will build a coalition to "degrade and destroy" the group.

• Steven Sotloff beheading shown in video, ISIS claims


• NATO leaders' focus could be split between ISIS and Russia


• Why ISIS beheaded James Foley


• ISIS: 5 things to know about the Iraqi jihadist group

Obama still did not give a timeline for deciding on a strategy to go after the extremist group's operations in Syria. "It'll take time to roll them back," the president said at a news conference during a visit to Europe.

Steven Sotloff, 31, was a freelance journalist working in Syria. (Reuters)

Obama's comments came Wednesday after he said the United States had verified the authenticity of a video released Tuesday showing the beheading of freelance reporter Steven Sotloff, two weeks after journalist James Foley was similarly killed.

Obama vowed the U.S. would not forget the "terrible crime against these two fine young men."

"Our reach is long, and justice will be served," Obama said.

Sotloff also held Israeli citizenship, Israel said on Wednesday after apparently withholding the information in a bid to stem the risks to the captive. He was also reported to be the grandson of two holocaust survivors.

In the Sotloff video, a masked militant warns Obama that as long as U.S. airstrikes against the militant group continue, "our knife will continue to strike the necks of your people."

Obama responded that he will continue to fight the militant threat and the "barbaric and ultimately empty vision" it represents.

"Our objective is to make sure that [ISIS] is not an ongoing threat to the region," he said. "And we can accomplish that. It's going to take some time and it's going to take some effort."

Sotloff, a 31-year-old Miami-area native who freelanced for Time and Foreign Policy magazines and for the Christian Science Monitor, vanished a year ago in Syria and wasn't seen again until he appeared in the video that showed Foley's beheading.

Dressed in an orange jumpsuit against an arid Syrian landscape, Sotloff was threatened in that video with death unless the U.S. stopped airstrikes on the Islamic State.

In the video distributed Tuesday and titled "A Second Message to America," Sotloff appears in a similar jumpsuit before he is apparently beheaded by a fighter with the Islamic State, the extremist group that has conquered wide swaths of territory across Syria and Iraq and declared itself a caliphate.

British Foreign Secretary Philip Hammond told the BBC Wednesday that the masked, British-accented jihadist appears to be the same person shown in the Foley footage.

In the video, the organization threatens to kill another hostage, this one identified as a British citizen.

Last week, Sotloff's mother, Shirley Sotloff, pleaded with his captors for mercy, saying in a video that her son was "an innocent journalist" and "an honourable man" who "has always tried to help the weak."

'They failed because, like people around the world, Americans are repulsed by their barbarism.'- President Barack Obama

Obama said the prayers of the American people are with the family of the "devoted and courageous journalist" who deeply loved the Islamic world and whose "life stood in stark contrast to those who murdered him so brutally."

"Whatever these murderers think they will achieve by murdering innocents like Steven, they have already failed," Obama said.

"They failed because, like people around the world, Americans are repulsed by their barbarism. We will not be intimidated. Their horrific act only unite us as a country and stiffen the resolve to take the fight against these terrorists. Those who make the mistake of harming Americans will learn we will not forget."

State Department spokeswoman Jen Psaki said Tuesday that it is believed that "a few" Americans are still being held by the Islamic State. Psaki would not give any specifics, but one is a 26-year-old woman who was kidnapped while doing humanitarian aid work in Syria, according to a family representative who asked that the hostage not be identified out of fear for her safety.