Jihadists have released a video apparently showing the beheading of an American journalist kidnapped in Syria, in the most direct retaliation yet to nearly two weeks of US air strikes on Iraq.

As calls mounted for Washington to expand its military intervention against the Islamic State of Iraq and the Levant or ISIL, the jihadist group threatened to kill another US reporter, Steven Sotloff, if bombings did not stop.

We have never been prouder of our son Jim. He gave his life trying to expose the world to the suffering of the Syrian people.

The nearly five-minute video titled A Message to America posted on the internet on Tuesday showed a masked militant beheading a man resembling James Foley, who has been missing since he was seized in Syria in November 2012.

It was distributed online by sources of the group known as ISIS and Islamic State, the group declares that Foley was killed because Obama ordered air strikes against IS in northern Iraq.

• Journalist beheaded by militants in Iraq
• Advisor: What’s happening in Iraqi turmoil

“We have never been prouder of our son Jim.”

“We have never been prouder of our son Jim. He gave his life trying to expose the world to the suffering of the Syrian people,” Foley’s mother Diane said in a Facebook message to supporters.

“We implore the kidnappers to spare the lives of the remaining hostages. Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world.”

Search to identify jihadist

The White House said US intelligence was studying the video, and that President Barack Obama had been briefed on it as he flew from Washington to resume his vacation on Martha’s Vineyard.

“If genuine, we are appalled by the brutal murder of an innocent American journalist and we express our deepest condolences to his family and friends,” National Security Council spokeswoman Caitlin Hayden said.

British Prime Minister David Cameron broke his holiday for talks on the threat posed by ISIL jihadists, announcing his plans in the following tweet:

If true, the murder of James Foley is shocking and depraved. I will today chair meetings on the situation in Iraq/Syria.

— David Cameron (@David_Cameron) August 20, 2014

Downing Street said in a statement on Wednesday that Cameron would meet with Foreign Secretary Philip Hammond and other senior officials “to discuss the situation in Iraq and Syria and the threat posed by ISIL (Islamic State of Iraq and the Levant, or ISIS, also Islamic State) terrorists”.

British intelligence agencies were reported to be using voice recognition software to try to identify the masked jihadist with a British accent who beheaded Foley.

British Secretary of State Philip Hammond voiced his “absolute horror” at the apparent beheading, which he said is “just is one more example in a catalogue of brutality by this organisation”, referring to ISIL.

“On the face of it, it appears to have been a British person. We’ll have to do some more analysis to make quite certain that that is the case,” Hammond said.

He said “atrocities” carried out by ISIL as it has seized swathes of territory in Iraq and Syria were driving the international community’s efforts to support Iraqis and Kurds in pushing back against the “evil organisation”. 

Danger didn’t stop Foley

Foley was an experienced correspondent who had covered the war in Libya before heading to Syria to follow the revolt against Bashar al-Assad’s regime, contributing to news site GlobalPost, Agence France-Presse (AFP) and other media outlets.

• Danger didn’t stop Foley from doing job he loved

AFP chairman Emmanuel Hoog described Foley “as a brave, independent and impartial journalist” whose work in Syria and other war zones was “widely admired”.

According to witnesses, Foley was seized in the northern Syrian province of Idlib on November 22, 2012. He has written for several US newspapers and magazines, including Time, Foreign Policy and The Christian Science Monitor.

Twitter campaigns not to share video

A campaign has been launched on Twitter urging people not to share the video in which the apparent execution is carried out in an open desert area with no immediate signs as to whether it is in Iraq or Syria.

Celebrities including actress Mia Farrow were among those who urged their followers to shun the horrific beheading footage, which the Islamic State of Iraq and the Levant (ISIL, also known as ISIS and Islamic State) claimed showed the death of the 40-year-old.

Blackout on group that murdered James Foley. Don’t share video. Give them nothing. #RespectJamesFoley — mia farrow (@MiaFarrow) August 20, 2014

Thousands of ordinary users from around the world have also backed the plan to deny ISIL publicity in horror at what they have done, using the hashtag #ISISmediablackout.

Twitter’s chief executive, Dick Costolo, said the firm was taking action against accounts which spread the video.

He wrote: “We have been and are actively suspending accounts as we discover them related to this graphic imagery. Thank you.”

Not only will I not share the video or any photos of James Foley’s murder, I will not watch it either. I can offer only prayers & privacy. — Hend (@LibyaLiberty) August 19, 2014

Instead of the graphic video, large numbers of people were sharing pictures of Foley smiling at work, encouraging people to disseminate them in his memory instead.