"We must do everything we can to defeat" ISIS

CAIRO -- The Islamic State of Iraq and Syria 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 ISIS militants were committed to inflicting horror for horror's sake.

Asked whether he believed fighters with ISIS, also known as the Islamic State of Iraq and the Levant, or ISIL, 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.

"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."

CBS News correspondent Charlie D'Agata reports that Henning was an aid worker, a taxi driver and a father of two whose only motivation was to cross into Syria to help Muslim victims in desperate need.

On Saturday, Henning's wife Barbara Henning said in a statement that the family was "devastated" by the news of his death.

"There are few words to describe how we feel at this moment," Barbara Henning said. "Myself, Lucy and Adam, and all of Alan's family and friends are numb with grief."

Henning, who is 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 ISIS 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 ISIS positions in Iraq, but not Syria.

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, Islam's annual "festival of sacrifice."

"Millions should be praying today for Alan Henning, a good and honorable 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, Lord Dannatt, called for British air power to be deployed in Syria as well as Iraq. "Dealing with half a problem is not going to solve the problem," Dannatt said.

The video mirrored other beheading videos shot by ISIS, and ended with a militant threatening a 26-year-old American hostage, Peter Kassig, who changed his name to Abdul-Rahman while in captivity.

"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 spokesperson Caitlin Hayden confirmed that ISIS 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.

This is the fourth such video released by ISIS. 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 enlisted in the U.S. Army in 2004, served in the 75th Ranger Regiment, a special operations unit, was deployed to Iraq in 2007 and medically discharged later that year at the rank of private first class.

His parents, Ed and Paula Kassig, called 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.

ISIS 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.

ISIS 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.