Authorities were on Monday investigating the suspected involvement of Western jihadists in the brutal video by the Islamic State group claiming the beheading of US aid worker Peter Kassig.

The killing of Kassig and the simultaneous beheadings of at least 18 Syrian military personnel in the video sparked global horror, with US President Barack Obama calling it "an act of pure evil".

It was the latest in a series of atrocities by IS, a Sunni Muslim extremist group that has seized control of large parts of Iraq and Syria.

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 a Briton, an Australian and a Dane.

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 IS video released on Sunday."

It added it was "possible" a second Frenchmen appeared in the video but said it was yet to confirm the individual's identity.

- Lured by online videos -

In July, Hauchard said in an interview with French television he had decided to join IS after watching videos online.

"The personal objective of everyone here is (to become a) shahid (martyr). That is the greatest reward," he said.

Thousands of foreign fighters have flocked to join IS 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 IS beheading videos and appeared again in Sunday's recording claiming Kassig's killing.

The father of another British jihadist fighting with IS initially told the media he had also seen his son in the video, but later said he was mistaken.

Britain's Foreign Office refused to comment on speculation about the identity of the fighters in the video, but a spokesman said: "We are analysing its contents."

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

"Abdul-Rahman was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity," Obama said.

In the undated video released on Sunday, the jihadist stands above a severed head he claims is Kassig's and challenges Obama to send more troops to the region to confront IS.

"Here we are burying the first American crusader in Dabiq, eagerly waiting for the remainder of your armies to arrive," the militant says, referring to a northern Syrian town.

Washington is preparing to double its military personnel in Iraq to up to 3,100 as part of the international campaign it is leading against the jihadists.

Kassig, an Iraq war veteran, had risked his life to provide medical treatment and relief supplies to those suffering from Syria's civil war.

- 'Unspeakable act of barbarism' -

Kassig's parents said they were "incredibly proud" of his humanitarian work to help Syrians trapped in a civil war.

"(He) lost his life as a result of his love for the Syrian people and his desire to ease their suffering," Ed and Paula Kassig said on Twitter.

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 IS on Monday, insisting the world would not be intimidated in the battle against it.

Sunday's video was substantially different from previous IS recordings of beheadings.

Kassig was not shown alive in the footage, and no direct threats were made against other Western hostages.

The video came as IS suffered battleground setbacks in Iraq supported by US-led air strikes, with government forces Saturday breaking the jihadists' months-long siege of the country's largest oil refinery.

Monitoring group the Syrian Observatory for Human Rights said Monday it had documented the execution of 1,429 people in Syria by IS in the five months since it declared the establishment of a "caliphate" in areas under its control.

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