Islamic State (IS) militants have released a video showing the beheading of a second American journalist, Steven Sotloff, and warns governments not to ally with the United States in its fight against the group.
The video, which emerged on September 2, shows Sotloff dressed in orange and on his knees in a desert landscape. A masked militant condemns U.S. attacks on IS, which has been targeted in air strikes in northern Iraq, and cuts the captive's throat.
Coming two weeks after IS issued a similiar video showing the beheading of another American journalist, James Foley, the video at once underscores the risks of U.S. involvement in battling Islamic State militants and increases pressure on President Barack Obama to step up action against them.
The U.S. National Security Council said on September 3 that it has determined that the video is authentic.
The confirmation came in a statement by National Security Council spokeswoman Caitlin Hayden.
U.S. President Barack Obama called Sotloff's killing a "horrific act of violence" and warned Islamic State militants that the reach of the United States is long and that "justice will be served."
British Prime Minister David Cameron condemned the beheading as an "absolutely disgusting, despicable act."
After the beheading, the militant in the latest video introduces a second captive, identified as David Haines and said to be British, and warns governments to stay out of "this evil alliance with America."
Sotloff had worked for the magazine "Time," as well as "The National Interest" and "Foreign Policy."
His mother made a video on August 27 urging IS leader Abu Bakr al-Baghdadi to release him.
"The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time," a spokesman for Sotloff's family, Barak Barfi, said after the release of the video.
In the video showing the killing of Foley, which was released on August 20, a masked man warned U.S. officials that Sotloff would be killed next if Washington did not end air strikes against IS militants in Iraq.
The United States has recently carried out dozens of air strikes against IS targets in Iraq.
The group and its allies control large parts of northern and western Iraq after entering the country at the start of this year from areas in northeastern Syria.
In a report released earlier on September 2, human rights watchdog Amnesty International said IS militants were guilty of "systematic ethnic cleansing" in northern Iraq.
The report cites "hair-raising" accounts from survivors of massacres that Amnesty said shows IS fighters have committed "war crimes."
The report said, "The massacres and abductions being carried out by the Islamic State provide harrowing new evidence that a wave of ethnic cleansing against minorities is sweeping across northern Iraq."
The report said just in the two villages of Qiniyeh on August 3 and Kocho on August 15 "the number of those killed...runs into the hundreds."