An Internet video released Friday purportedly shows an Islamic State fighter beheading captured British aid worker Alan Henning and threatening another American captive in the fourth such killing carried out by the militant group.

The video mirrored other beheading videos shot by the Islamic State group, which now holds territory along the border of Syria and Iraq. It ended with an Islamic State fighter threatening a man they identified as former Army Ranger Peter Kassig.

"Obama, you have started your aerial bombardment of Shams (Syria), which keeps on striking our people, so it is only right that we continue to strike the neck of your people," the masked militant in the video says.

U.S. officials told Fox News that they are analyzing the tape but had no reason to doubt its authenticity.

"The United States strongly condemns the brutal murder of United Kingdom citizen Alan Henning by the terrorist group ISIL," the White House said in a statement. 

"Mr. Henning worked to help improve the lives of the Syrian people and his death is a great loss for them, for his family and the people of the United Kingdom," the White House said. "Standing together with our UK friends and allies, we will work to bring the perpetrators of Alan's murder - as well as the murders of Jim Foley, Steven Sotloff and David Haines - to justice."

The video was released in the same manner as other Islamic State group videos and the masked militant sounded similar to the one who carried out the other killings.

The British Foreign Office had no immediate comment.

A spokesperson for the National Security Council confirmed in a statement that Kassig is being held by the Islamic State group. 

According to military records, Kassig, a former Army Ranger from Indianapolis, was deployed to Iraq from April to June 2007 and was medically discharged later that year.

He then went on to help Syrian refugees in a Lebanese hospital, TIME reports. Kassig also started his own aid group, Special Emergency Response and Assistance (SERA).

The group said on its Facebook page that it aims to provide “Syrian refugees in Lebanon and Syria with medical assistance, medical supplies, and clothing for refugees, and non-perishable food as well as cooking fuel, cooking stoves,” according to TIME.

It is not clear when or where Kassig was captured by militants.

The video released Friday begins with a British news report announcing the country’s decision to join the U.S.-led coalition that is launching airstrikes against ISIS.

It then switches to the executioner with the hostage on his knees. The hostage, purportedly Henning, says he blames his death on British policy while the executioner says his blood is on the hands of the British parliament.

The executioner then begins the process of beheading the man.

This is the fourth such video released by the Islamic State group. The full beheadings are not shown in the videos, but the British-accented, English-speaking militant holds a long knife and appears to begin cutting the three men, American reporters James Foley and Steven Sotloff and British aid worker David Haines.

Henning, 47, nicknamed "Gadget," had joined an aid convoy and was taken captive on Dec. 26, shortly after crossing the border between Turkey and Syria. Earlier this week, Henning's wife Barbara Henning asked the militants in a televised plea: "Please release him. We need him back home."

Dozens of Muslim leaders in Britain have urged the Islamic State group to release Alan Henning. His wife had said she had been given hope by "the outcry across the world" over her husband's imprisonment.

The Islamic State group has its roots in Al Qaeda's Iraqi affiliate but was expelled from the global terror network over its brutal tactics and refusal to obey orders to confine its activities to Iraq. It metamorphisized amid the bloody 3-year civil war in neighboring Syria, growing stronger to the point of being able to launch a lightning offensive across much of northern Iraq, routing security forces there.

The extremist group has been widely denounced by mainstream Muslim authorities.

Other foreigners are believed held by the Islamic State group. On Friday, the father of John Cantlie, a British photojournalist held by the group, appealed for his release in a video, saying he was a friend of Syria.

Fox News’ Catherine Herridge, Justin Fishel and The Associated Press contributed to this report.