The video showing the beheading of U.S. journalist Steven J. Sotloff appears to be authentic, U.S. officials said Wednesday as President Obama said America would not be “intimidated” by Islamic State militants.

“The U.S. intelligence community has analyzed the recently released video showing U.S. citizen Steven Sotloff and has reached the judgment that it is authentic,” said National Security Council spokeswoman Caitlin Hayden.

Sotloff’s family, through a spokesman, issued a statement suggesting they believed the footage to be authentic as well. “The family knows of this horrific tragedy and is grieving privately,” said the spokesman, Barak Barfi.

It was the second such killing carried out by Islamic State militants. Late last month, American journalist James Foley was beheaded in a similar videotaped slaying.

In Estonia, Obama vowed to “destroy” the Islamic State militants and said the United States would “not be intimidated” by the bloodshed.

Steven Sotloff had gone missing in Syria nearly two years ago. (Daily Caller via Reuters)

The Sotloff video, which surfaced online Tuesday, shows a bound and kneeling Sotloff being held at knife-point by a black-clad militant against a desert backdrop similar to the one shown in an earlier video depicting the beheading of Foley.

The gruesome killings and the group’s threat to decapitate other captives put new pressure on the Obama administration at a time when it is weighing how the United States should respond to an al-Qaeda offshoot that is seen as ruthless but that has not carried out plots beyond the region.

The United States has carried out more than 100 airstrikes against Islamic State fighters in Iraq over the past month since the group expanded its control of territory there. But the administration has avoided direct U.S. military involvement in Syria aside from a failed attempt to rescue Foley and other captives earlier this year.

The Pentagon and U.S. intelligence agencies have developed lists of possible Islamic State targets in Syria. But Obama, who has expressed concerns about being drawn into Syria’s civil war, indicated last week that he had not yet approved new strikes. “We don’t have a strategy yet,” Obama said in a remark that was later derided by Republican lawmakers and critics.

U.S. officials believe that as many as a dozen Americans may be fighting alongside the Islamic State in Iraq and Syria and that the group holds at least two other American hostages, both aid workers.

Sotloff, a 31-year-old freelance journalist, had written for Time magazine, Foreign Policy and other publications. He was believed to have been taken captive shortly after entering Syria in 2013, and he was shown in the video of Foley that the Islamic State released last month. Sotloff’s mother, Shirley, recently released a video pleading with the group for her son’s release.

In the video, an unidentified militant denounces the Obama administration, saying, “I’m back, Obama, and I’m back because of your arrogant foreign policy towards the Islamic State.” He cites recent U.S. airstrikes against the group in Iraq and warns that “just as your missiles continue to strike our people, our knife will continue to strike the necks of your people.”

White House press secretary Josh Earnest said Tuesday that he could not confirm reports of the execution of American journalist Steven Sotloff but that the administration's "thoughts and prayers first and foremost are with Mr. Sotloff and Mr. Sotloff's family and those who worked with him." (AP)

The militant speaks with a British accent, raising the possibility that he is the same person who killed Foley, 40. Both hostages wore orange jumpsuits, much like prisoners held by the United States at Guantanamo Bay, Cuba.

Foley and Sotloff may not have been killed at the same time or place. The militant in the Sotloff video appears to refer to U.S. airstrikes at Amerli in Iraq, which began over the weekend. Sotloff also appears to have more hair on his head and face than in the first video.

The latest video, obtained by the Site Intelligence Group, an organization that tracks the online postings of Islamist militant groups, also shows another hostage identified as David Cawthorne Haines. Haines is a British aid worker who was abducted in March 2013 near the Atmeh refu­gee camp adjoining the Turkish border and the Syrian province of Idlib, according to aid workers involved in efforts to secure his release.

In an apparent reference to Britain, Sotloff’s killer warns “governments that enter this evil alliance of America . . . to back off and leave our people alone.” Haines is shown in the same kneeling position and wearing the same type of orange jumpsuit as Sotloff and Foley before him.

Sotloff had traveled into and out of Syria several times before he was abducted after entering the country from Turkey on Aug. 4, 2013, according to a person familiar with the details of his case. The Florida native had covered turmoil in countries including Egypt and Libya, where his reporting served as the basis of a Time magazine reconstruction of the Sept. 11, 2012, attacks on U.S. compounds in Benghazi.

In the video that surfaced Tuesday, Sotloff stares stone-faced into the camera while delivering a statement apparently scripted by his captors. “I am Steven Joel Sotloff,” he says, adding that he is “paying the price” for the Obama administration’s policies in the Middle East.

The video lasts less than three minutes but appears to have been carefully produced by the terrorist group, which has used online propaganda to position itself as a successor to al-Qaeda and declare itself the leader of a new Islamic caliphate.

The recording opens with footage of Obama’s remarks after Foley’s death, then cuts to video of Sotloff taken from two camera angles. A digital representation of the Islamic State’s black banner flutters in the upper left corner of the frame.

Foley’s killing last month prompted an unusually emotional response from Obama, who interrupted his vacation on Martha’s Vineyard to warn that the United States would be “relentless” against the Islamic State. Obama, who departed Tuesday for a trip to Estonia and Wales, did not publicly respond to Sotloff’s apparent killing.

British Prime Minister David Cameron issued a statement saying that the video, if authentic, depicts “a despicable and barbaric murder” and that the Islamic State fighters “threaten Syrians, Iraqis, Americans and British people alike and make no distinction between Muslims, Christians or any other faith.”

Haines is among several Western hostages believed to be held by the Islamic State, possibly in Raqqah, a Syrian city that serves as the group’s stronghold.

U.S. and British authorities are still seeking to determine the identity of Foley’s killer. His British accent has intensified concerns in Britain and elsewhere in Europe about the large number of European citizens who have flocked to Syria’s civil war over the past four years.

Britain, France and other countries have seen hundreds of citizens depart for Syria, almost all of them carrying passports that would allow them to return to their home countries or travel to other Western nations.

British officials raised the country’s threat level to “severe” last week as the nation’s home secretary warned that terrorist plots were seen as “highly likely” because of the fallout from the conflicts in Iraq and Syria.

Adam Goldman, Anne Gearan, Julie Tate and Karen DeYoung in Washington and Liz Sly in Beirut contributed to this report.