The extremists who have beheaded another Western hostage are deaf to reason and must be destroyed, British Prime Minister David Cameron said Saturday.
Cameron, speaking after a security briefing at his rural retreat Chequers, said Friday's slaying of 47-year-old British aid worker Alan Henning demonstrated that the militants known as the Islamic State of Iraq and Syria are committed to inflicting horror for horror's sake.
Asked whether he believed ISIS fighters would kill more hostages, Cameron said they would have to be hunted down to be stopped. He declined to say whether Britain would extend its involvement in U.S.-led airstrikes on ISIS to Syria, where the hostage killings are believed to have happened.
Henning, a taxi driver from the town of Eccles in northwestern England, was abducted minutes after his aid convoy entered Syria on Dec. 26. He was the fourth Western hostage to be killed by ISIS since mid August, after two American journalists and another British aid worker. In their latest video, Henning's killers linked their action to a Sept. 26 vote in the British Parliament to deploy the Royal Air Force against ISIS in Iraq.
The video mirrored other beheading videos shot by ISIS and ended with a militant threatening a 26-year-old American hostage, Peter Kassig.
"(President Barack) Obama, you have started your aerial bombardment of Sham (Syria) which keeps on striking our people, so it is only right that we continue to strike the necks of your people," the masked militant in the video said.
National Security Council spokeswoman Caitlin Hayden confirmed that ISIS militants had Kassig.
In Indianapolis, Kassig's parents released a video on Saturday, pleading with their son's captors to free him.
In the family's video, Ed Kassig says his son, who now goes by the first name Abdul-Rahman after converting to Islam during his captivity, was captured on Oct. 1, 2013, in Syria, where he was providing aid for refugees fleeing that country's civil war.
"We implore those who are holding you to show mercy and use their power to let you go," Paula Kassig said in the video, holding a photo of her son.