BEIRUT/BAGHDAD - Authorities were on Monday investigating the suspected involvement of Western militants 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 Normandy 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."

An investigation into Hauchard was opened in August by French authorities "for criminal association in relation to a terrorist organisation," a judicial source said.

Interviewed in July by French TV station BFM via Skype, Hauchard said 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.

An intelligence source said it was also being verified whether a second French citizen was among the militants seen in the video. 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 militants.

A British-accented militant 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 militant 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 a statement.

In the undated video released on Sunday, the militant stands above a severed head he claims is Kassig's and urges 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 militants.

Monitoring group the Syrian Observatory for Human Rights said the militant Islamic State group (IS) has executed nearly 1,500 people in Syria in the five months since it declared the establishment of a "caliphate"..

"The Syrian Observatory for Human Rights has documented the execution of 1,429 people since the IS announced its 'caliphate' in June," the group's director, Rami Abdel Rahman, said. The majority of IS's victims in Syria have been civilians, he said.

Meanwhile in Iraq, two car bombs exploded in the Iraqi capital on Monday, killing at least 12 people and wounding at least 29, security and medical officials said.

One exploded on a main commercial street in the Mashtal area of east Baghdad, while the other hit a similar road in Amriyah in the capital's west. The city is hit by near-daily bombings and shootings that kill hundreds of people each month.

And security forces, despite being deployed at checkpoints and other positions across the city, are consistently unable to prevent the attacks.