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 issued 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.
U.S. State Department spokeswoman Jen Psaki said officials are checking into the authenticity of the video. If genuine, Psaki said, "we are sickened."
The images follows the video release of the gruesome beheading of U.S. freelance journalist Jim Foley Last month.
White House spokesman Josh Earnest could not confirm the beheading, but told reporters "our thoughts and prayers" are with Sotloff's family.
Sotloff was taken captive in Syria in August 2013, roughly a year after Foley was taken hostage, and worked for Time and Foreign Policy magazines.
The video had clear echoes of Foley's murder. Sotloff, as with Foley, appeared in an orange jumpsuit, and his killer is dressed in black, his face hidden.
Similar to Foley, Sotloff was 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 to conduct airstrikes to disrupt the ability of the Islamic State group to put U.S. personnel and facilities at risk, or to 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, including mass summary killings and abductions.
Republican Congressman Ed Royce and Democratic Congressman Eliot Engel condemned the killing and called on the formation of an international coalition to defeat the extremist group.
Engel warned, "If we don't disrupt and defeat it, they will attack homelands all over the world." The U.S. lawmaker is the ranking Democrat of the House Foreign Affairs Committee.
Royce, Chairman of the House Foreign Affairs Committee, issued this statement:
“I am disgusted by what appears to be another horrendous killing of an American journalist at the hands of ISIS. My thoughts and prayers are with Steven Sotloff’s family. Sadly, ISIS is bringing this barbarity across the region – beheading and crucifying those who don’t share their dark ideology. 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.”
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.