BEIRUT -- Islamic State extremists released a video Tuesday purportedly showing the beheading of a second American journalist, Steven Sotloff, and warning U.S. President Barack Obama that as long as U.S. airstrikes against the militant group continue, "our knife will continue to strike the necks of your people."
The footage -- depicting what the U.S. called a sickening act of brutality -- was posted two weeks after the release of video showing the killing of James Foley and just days after Sotloff's mother pleaded for his life.
Barak Barfi, a spokesman for the family, said the Sotloffs had seen the video but authorities have not established its authenticity.
'Appalled to learn of the barbaric and unacceptable death of Steven Sotloff at the hands of ISIL'
"The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time," Barfi said.
Sotloff, a 31-year-old Miami-area native who freelanced for Time and Foreign Policy magazines, vanished in Syria in August 2013 and was not seen again until he appeared in a video released last month 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.
In the video, the organization threatens to kill another hostage, this one identified as a British citizen, David Cawthorne Haines. It was not immediately clear who Haines was.
Prime Minister Stephen Harper tweeted from London his revulsion at the apparent killing.
"Appalled to learn of the barbaric and unacceptable death of Steven Sotloff at the hands of ISIL," Harper tweeted. "Our thoughts and prayers are with his loved ones."
Britain and France called the killing "barbaric." In Washington, State Department spokeswoman Jen Psaki said U.S. intelligence analysts will work as quickly as possible to determine if the video is authentic.
"If the video is genuine, we are sickened by this brutal act, taking the life of another innocent American citizen," Psaki said. "Our hearts go out to the Sotloff family."
Psaki said it is believed "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 kidnapped while doing humanitarian-aid work in Syria, said a family representative who asked the hostage not be identified out of fear for her safety.
The fighter who apparently beheaded Sotloff in the video called it retribution for Obama's continued airstrikes against the group.
"I'm back, Obama, and I'm back because of your arrogant foreign policy toward the Islamic State... despite our serious warnings," the fighter says. "So just as your missiles continue to strike our people, our knife will continue to strike the necks of your people."
The killer specifically mentions the recent U.S. airstrikes around the Mosul dam and the beleaguered Iraqi town of Amirli, making it unlikely Sotloff was killed at the same time as Foley, as some analysts had speculated.
Over the weekend, Iraqi government forces with help from U.S. airstrikes broke the Islamic State's two-month siege of Amirli, a town where some 15,000 Shiite Turkmens had been stranded.
The SITE Intelligence Group, a U.S. terrorism watchdog, first reported the video's existence.
-- The Associated Press