The footage, seen by AFP after it was found online
by private terrorism monitor SITE, shows 31-year-old freelance reporter
Steven Sotloff dressed in orange and on his knees in a desert landscape.
The
masked militant addresses US President Barack Obama, condemns air
strikes against the Islamic State -- a Sunni jihadist group that
operates in Iraq and Syria -- and cuts Sotloff's throat.
He then introduces a second captive, identified in a caption by name as a British citizen.
"I'm
back, Obama, and I'm back because of your arrogant foreign policy
towards the Islamic State," the militant says, speaking in British
English with a London accent.
This was an apparent reference to a
previous video in which US journalist James Foley was murdered, again by
a suspected British foreign fighter and in an almost identical fashion.
British Prime Minister David Cameron said the latest video depicted an "absolutely disgusting, despicable act."
The previous video was released last month after Obama ordered air strikes against the Islamic State.
It
showed 40-year-old Foley's death at the hands of a masked militant who
then threatened Sotloff, a Miami-born freelance reporter who has written
for Time magazine, Foreign Policy and other outlets.
US
officials said they were working to confirm the authenticity of the
latest video, and State Department spokeswoman Jen Psaki said Washington
would be "sickened" if it proved genuine.
American air strikes
against IS forces continued in the wake of Foley's death, and more than
120 have been carried out, most of them hitting targets around Iraq's
largest dam north of Mosul.
The video, shot with the same high production values as its predecessor, is entitled: "A second message to America."
It opens with a clip of Obama vowing to be "relentless" in his determination to protect US citizens from IS attacks.
Sotloff
identifies himself in English and calmly explains that he is paying the
price for Obama's policy. His killer also speaks in English.
Sotloff
was kidnapped in northern Syria more than a year ago, on August 4,
2013. His family only publicly revealed his captivity last month, having
previously requested a media blackout.
The
reporter's former employers paid tribute to a man widely respected for
his intrepid reporting in Syria and the wider region, including a
previous stint in Libya.
"We are shocked and deeply saddened by reports of Steven Sotloff's death," said Nancy Gibbs, editor of Time.
"Steven
was a valued contributor to Time and other news organizations, and he
gave his life so readers would have access to information from some of
the most dangerous places in the world."
Sharing
on Twitter a piece Sotloff had once written from a besieged rebel
enclave in the Syrian city of Aleppo, Foreign Policy's Middle East
editor David Kenner dubbed him "a brave and talented reporter."
Sotloff's
family lives in Miami. When the threat to his life became public, his
mother Shirley Sotloff made a direct video appeal to IS commander Abu
Bakr al-Baghdadi to spare his life