Intelligence agencies are investigating the involvement of Western jihadists, including possibly an Australian, in the beheading of aid worker Peter Kassig and 18 Syrian soldiers.

The killing of Kassig and the simultaneous beheadings of at least 18 Syrian military personnel in the video showed the Syrian men kneeling on the ground each before a separate executioner, whose faces were uncovered.

Among the militants shown beheading the Syrian servicemen were some known foreign fighters, including at least one Frenchman and possibly an Australian, a Briton and a Dane.

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French authorities identified one of the executioners as Maxime Hauchard, a 22-year-old from a small village in northern France who left for Syria in August last year.

The Paris prosecutor’s office said “circumstantial evidence confirms the involvement of a Frenchman in the decapitation of Syrian prisoners shown in an ISIL video released on Sunday”.

It added it was “possible” a second Frenchman appeared in the video but said it was yet to confirm the individual’s identity.

Thousands of foreign fighters have flocked to join ISIL in Iraq and Syria, and experts say they are often among the most violent and brutal of the jihadists.

A British-accented jihadist has been at the centre of previous ISIL beheading videos and appeared again in Sunday’s recording claiming Kassig’s killing.

The video sparked global horror, with US President Barack Obama calling it “an act of pure evil”.

Obama has announced a review of how Washington can release American hostages, just 24 hours after the video’s release.

In a letter from Obama, dated last Tuesday, Christine Wormuth, the US undersecretary of defence for policy, says the review will focus “on examining family engagement, intelligence collection and diplomatic engagement policies”.

“The president recently directed a comprehensive review of the US government policy on overseas terrorist-related hostage cases,” Wormuth said in the note posted on The Daily Beast news site.

Kassig, who took the name Abdul-Rahman after converting to Islam, was captured last year and became the fifth Western hostage beheaded by ISIL after the two US reporters and two British aid workers.

On Monday, the parents of the 26-year-old paid tribute to their son and said they would try to “forgive” the jihadists.

Kassig’s parents called for healing and prayer as they mourned their loss.

“Please allow our small family the time and privacy to mourn, cry and – yes – forgive and begin to heal,” Peter’s father Ed said in an emotional address outside his church.

“Please pray for Abdul-Rahman, or Pete if that’s how you knew him, at sunset this evening. Pray also for all people in Syria, in Iraq and around the world that are held against their will.”

Peter’s mother Paula said while their world had been torn apart by the death of their son, they would focus on healing.

“Rather than letting the darkness overwhelm him he has chosen to believe in the good, in himself and in others…. One person makes a difference,” she said.

“Our hearts are battered. But they will mend. The world is broken, but it will be healed in the end.”

In Kassig’s home state of Indiana, Governor Mike Pence called the killing “an unspeakable act of barbarism”.

US Secretary of State John Kerry also used the word “barbarism” to describe ISIL on Monday, insisting the world would not be intimidated in the battle against it.

– AAP