CAIRO, Egypt -- The Islamic State extremists who have beheaded another Western hostage are deaf to reason and must be destroyed, British Prime Minister David Cameron said Saturday as Muslims worldwide were urged to pray for the victim on one of Islam's holiest days.
Cameron, speaking after a security briefing at his rural retreat Chequers, said Friday's slaying of 47-year-old English aid worker Alan Henning demonstrated that Islamic State militants were committed to inflicting horror for horror's sake.
Asked whether he believed Islamic State 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 the Islamic State group to Syria, where the hostage killings are believed to have happened.
"The fact that this was a kind, gentle, compassionate and caring man who had simply gone to help others, the fact they could murder him in the way they did, shows what we are dealing with," Cameron said. "This is going to be our struggle now. ... We must do everything we can to defeat this organization."
Henning, a taxi driver from the town of Eccles in northwest England, was abducted minutes after his aid convoy entered Syria on Dec. 26. He was the fourth Western hostage to be killed by Islamic State since mid-August, following two American journalists and another British aid worker. In their latest video, Henning's killers linked their action to a vote Sept. 26 in British Parliament to deploy the Royal Air Force against Islamic State positions in Iraq, but not Syria.
Henning's widow, Barbara, said the family was devastated at the loss of a "decent, caring human being" and that they were proud of the work he did in helping others.
"There are few words to describe how we feel at this moment ... all of Alan's family and friends are numb with grief," she said in a statement through Britain's Foreign Office.
Muslim leaders across Britain urged worshippers worldwide to pray for Henning and peace in the Middle East as they gathered at mosques to celebrate Eid al-Adha, or "Feast of Sacrifice," a major holiday in Islam.
"Millions should be praying today for Alan Henning, a good and honourable man," said Muslim peace activist Shaukat Warraich, speaking outside a mosque in the central English city of Birmingham.
Egypt's Foreign Ministry denounced what it called a "barbaric and savage act that fully contradicts Islamic religion tenets and the simplest human and ethical rules."
Britain's former army chief of staff, Gen. Richard Dannatt, called for British air power to be deployed in Syria as well as Iraq, but not for Western ground forces. "This is a fight for the soul of Islam. This is their fight," Dannatt said, pointing to Jordan and Turkey as countries which need "to get stuck into this fight."
Farooq Siddique, former leader of a British government initiative to combat extremism, said Western involvement plays into the extremists' hands. "Saudi Arabia has 700 jets. They are using only 10 of them. Why do they need the West to go and help them?" Siddique said.
The video mirrored other beheading videos shot by the Islamic State group, and ended with a militant threatening a 26-year-old American hostage, Peter Kassig.
"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 Islamic State militants had Kassig.
"We will continue to use every tool at our disposal -- military, diplomatic, law enforcement and intelligence -- to try to bring Peter home to his family," Hayden said.
In Indianapolis, Kassig's parents released a video message 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 Kassing said in the video, holding a photo of her son.
The militant video on Friday was the fourth such video released by the Islamic State group. Previous victims were American reporter James Foley, American-Israeli journalist Steven Sotloff and British aid worker David Haines.
FBI Director James Comey says American officials believe they know the identity of the masked militant, who speaks in a London accent. Comey has declined to name the man or reveal his nationality.
According to his military record, Kassig served in the U.S. Army's 75th Ranger Regiment, a special operations unit, was deployed to Iraq from April to July 2007 and medically discharged later that year at the rank of private first class.
On Friday, Ed and Paula Kassig issued a statement calling for the world to pray for their son.
They said Kassig had been working for the relief organization he founded, Special Emergency Response and Assistance, or SERA, when he was captured a year ago on his way to Deir Ezzour in eastern Syria. He converted to Islam while in captivity and the family has heard from former hostages that his faith has provided him comfort.
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 grew more extreme and powerful amid the 3-year civil war in Syria, launching a lightning offensive this summer that captured territory in both countries.
Islamic State militants may hold many more hostages. On Friday, the father of John Cantlie, a British photojournalist held by the group, appealed for his release in a video, describing his son as a friend of Syria.
