This undated family photo shows Alan Henning of Britain, who was a hostage of the Islamic State. (AP)

A new Islamic State video released Friday depicts the beheading of a 47-year-old British aid worker and shows another potential victim, an American hostage who is a former U.S. Army Ranger and veteran of the Iraq war.

The killing of Alan Henning marked the fourth video in which the same Islamic State militant has decapitated a hostage. The group has also killed two American journalists and another British aid worker.

In the latest video, the militant, dubbed “Jihadi John,” threatens to kill the American, Peter Edward Kassig, 26, who disappeared Oct. 1, 2013, on his way to Deir al-Zour in eastern Syria.

“Obama, you have started your aerial bombardment in Sham [a reference to greater Syria], which keeps on striking on our people,” the militant says. “So it’s only right we continue to strike the necks of your people.”

British Prime Minister David Cameron tweeted late Friday that “the brutal murder of Alan Henning by ISIL shows just how barbaric these terrorists are. My thoughts are with his wife and their children.” ISIL is an acronym for the group.

The parents of Peter Kassig, an American aid worker being held hostage by Islamic State militants, released a statement pleading for their son's release. (YouTube/Kassig Family)

“The United States strongly condemns the brutal murder of United Kingdom citizen Alan Henning by the terrorist group ISIL,” President Obama said in a statement. “Standing together with a broad coalition of allies and partners, we will continue taking decisive action to degrade and ultimately destroy ISIL.”

Dressed in an orange jumpsuit, Kassig does not speak in the video. He served in Iraq in 2007 for about four months before receiving a medical discharge. Later that year he returned to the United States to pursue training as an emergency medical technician.

In a statement, his parents, Ed and Paula Kassig, said: “We ask everyone around the world to pray for the Henning family, for our son, and for the release of all innocent people being held hostage in the Middle East and around the globe.”

In a 2012 interview with CNN, Kassig said he traveled to Lebanon because of a desire to help those suffering in Syria’s civil war.

“We each get one life and that’s it,” Kassig said in the interview. “The way I saw it, I didn’t have a choice. This is what I was put here to do. I guess I am just a hopeless romantic, and I am an idealist, and I believe in hopeless causes.”

Former hostages told the Kassig family that their son converted to Islam in captivity and had changed his name to Abdul-Rahman.

“The family understands from speaking to former hostages that Kassig’s faith has provided him comfort during his long captivity,” a family spokeswoman said.

Lisa Monaco, assistant to the president for homeland security and counterterrorism, said that if the video purportedly showing an Islamic State militant beheading British hostage Alan Henning is verified, it is a "very clear example of the brutality of this group." (AP)

The Islamic State is holding another American, a woman who traveled to Syria on a humanitarian mission. The woman’s family and the FBI have asked the news media not to identify her for fear of putting her at greater risk.

The FBI says it thinks it has identified the man responsible for the beheadings of American journalists James Foley and Steven Sotloff. The same man appears to have killed British aid worker David Haines and Henning.

Henning, a taxi driver from the northern English city of Manchester, disappeared in Syria nine months ago while delivering aid as part of a charity convoy.

Henning’s wife, Barbara, had made several appeals to the Islamic State to let her husband go, asking for mercy based on “Alan’s humanitarian motives for going to Syria.”

News of Henning’s apparent killing came just after the father of another British hostage, journalist John Cantlie, pleaded for his release “to those he loves and who love him.”

Cantlie has appeared in three videos in which he speaks from behind a desk, with a black backdrop, and criticizes U.S. and British policies in the Middle East.

Julie Tate contributed to this report.