An Internet video uploaded Tuesday by Islamic State purports to show the beheading of U.S. journalist Steven Sotloff.
A masked figure in the video also issue a threat against a British hostage, a man the group named as David Haines, and warned governments to back off "this evil alliance of America against the Islamic State," the SITE monitoring service reported.
The purported killer addresses President Barack Obama, saying Sotloff's death was in retaliation for U.S. airstrikes against the militants.
The militant says, "Just as your missiles continue to strike our people, our knife will continue to strike the necks of your people."
U.S. State Department spokeswoman Jen Psaki said officials are checking into the video's authenticity. If it's genuine, Psaki said, "we are sickened."
The images follow last month's video release of the gruesome beheading of U.S. freelance journalist Jim Foley.
Video appears authentic
A source familiar with the matter said that while U.S. officials have yet to formally confirm the validity of the video, it appeared to be authentic.
Iraq's outgoing foreign minister Hoshiyar Zebari, condemned what he called "this savage killing...an example of savagery and evil," and he said this was evidence of the need for Iraq and the West to defeat the Islamic State.
"We have a common enemy and the whole world is moving in the right direction to stop this savagery and brutality," said Zebari. "The whole world is standing united against IS. They must be defeated so these horrid scenes will not be repeated."
Iraqi Shi'ite Muslim politician Sami Askari, who is close to outgoing Prime Minister Nuri al-Maliki, said, "They are trying to scare the Americans not to intervene. I don't think Washington will be scared and stop. This is evil. Every human being has to fight this phenomenon. Like cancer, there is no cure. You have to fight it."
White House spokesman Josh Earnest could not confirm the execution, but told reporters "our thoughts and prayers" are with Sotloff's family.
British Prime Minister David Cameron called Sotloff's murder "disgusting and despicable."
Sotloff was taken captive in Syria in August 2013, roughly a year after Foley was taken hostage. He worked for Time and Foreign Policy magazines.
The video has clear echoes of Foley's murder. As with Foley, Sotloff appears in an orange jumpsuit; his killer is dressed in black, his face hidden.
Like Foley, Sotloff is forced to kneel in what appears to be a strip of desert before being beheaded.
It was unclear when the video was shot.
The Islamic State group has seized a large swath of land across eastern Syria and northwestern Iraq, declaring the area a "caliphate."
The United States has been conducting airstrikes in coordination with Iraq's government against the group since early August.
Pentagon Press Secretary Rear Admiral John Kirby said Tuesday the U.S. would continue airstrikes to disrupt the Islamic State group's ability to harm U.S. personnel and facilities. The raids also support further humanitarian aid to those persecuted by the extremist group.
Amnesty International on Tuesday said it had evidence Islamic State militants are conducting a systematic campaign of ethnic cleansing in northern Iraq, carrying out war crimes that include mass summary killings and abductions.
U.S. Congressmen Ed Royce and Eliot Engel condemned the killing and called for the formation of an international coalition to defeat the extremist group.
"If we don't disrupt and defeat it," Engel warned, its members "will attack homelands all over the world." The U.S. lawmaker is the ranking Democrat of the House Foreign Affairs Committee.
Royce, the committee's chairman, issued a statement saying he was "disgusted by what appears to be another horrendous killing of an American journalist at the hands of ISIS." He used that acronym for the group, adding his sympathies to Sotloff's family.
"... Sadly, ISIS is bringing this barbarity across the region — beheading and crucifying those who don’t share their dark ideology," said Royce. "The threat from this group seems to grow by the day. Working with key allies, the United States needs to be acting urgently to arm the Kurds on the ground who are fighting them, and targeting ISIS from the air with drone strikes.”
Royce said his committee will hold a hearing on the threat posed by the Islamic State after Congress returns next week. Engel said that after the 60 days allotted in the War Powers Act is over, President Obama should come to Congress to ask for the authorization of more airstrikes against Islamic State targets in Iraq.
A Sotloff family spokesman said it is aware of the video and is grieving privately.
On August 27, Sotloff's mother, Shirley Sotloff, issued a video plea to IS's leader during which she described her son as an "honorable man" who should not be punished for U.S. government actions.
"I've learned that Islam teaches that no individual should be held responsible for the sins of others," she said on the video posted on YouTube. "Steven has no control over the actions of the U.S. government. He is an innocent journalist. I've also learned that you, the Caliph, can grant amnesty. I ask you to please release my child."
Material for this report came from Reuters. Cindy Saine contributed reporting from Washington.