Steven Sotloff’s beheading marks the second American journalist brutally murdered by the Islamic State in less than two weeks. Mazen Mahdi / EPA

An video posted online Tuesday purported to show the beheading of U.S. journalist Steven Sotloff by the Islamic State, which called the action retribution for continued U.S. airstrikes in Iraq.

State Department spokeswoman Jen Psaki said of the video on Tuesday, if genuine, that "we are sickened." The Obama administration subsequently said on Wednesday that it believed the video was authentic.

In a choreographed video with a masked Islamic State soldier standing next to him, Sotloff describes himself as "paying the price" for U.S. intervention in Iraq with his life.

The Associated Press reported that Sotloff's family was aware of the video and is "grieving."

The 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 executioner appeared to be the same British-accented man who appeared in an Aug. 19 video showing the killing of American journalist James Foley, and the new video showed a similar desert setting. In both videos, the captives wore orange jumpsuits.

Sotloff, who freelanced for Time and Foreign Policy magazines, had last been seen in Syria in August 2013 until he appeared in a video released online by the Islamic State that showed the beheading of Foley. In that video, against the backdrop of an arid Syrian landscape, Sotloff was threatened with death unless the U.S. stopped airstrikes on the group in Iraq.

In the video distributed on Tuesday and titled "A Second Message to America," Sotloff is beheaded by an Islamic State fighter.

The killer specifically mentions the recent U.S. airstrikes around the Mosul dam and the beleaguered Iraqi town of Amirli, making it unlikely that 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.

In a statement Tuesday evening from U.S. Central Command, military officials said an airstrike conducted Monday against Islamic State militants near the Mosul Dam damaged or destroyed 16 armed vehicles.

The United States resumed airstrikes in Iraq in August for the first time since the end of the U.S. occupation in 2011. The raids followed major gains by the Islamic State, which has declared a caliphate in areas it controls in Syria and Iraq.

Last week Sotloff's mother, Shirley Sotloff, pleaded for his release in a video directed at the Islamic State. Her plea came as a U.N. commission accused the group, which dominates a swath of territory spanning the Syria-Iraq border, of committing crimes against humanity.

Al Jazeera and news wires