The mother of American journalist James Foley, apparently executed by Islamic State jihadists, on Wednesday paid tribute to her son who she said had died trying to expose the suffering of the Syrian people.

Condolences and shocked messages poured in after the Islamist group released a video showing a masked militant beheading a man resembling Foley, who has been missing since he was seized in Syria November 2012.

"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.

"We thank Jim for all the joy he gave us. He was an extraordinary son, brother, journalist and person."

A second captive, said to be US reporter Steven Sotloff, was shown alive in the video, along with a warning that his fate rests on US President Barack Obama ordering a halt to strikes against the jihadist group which seized swathes of Syria and Iraq.

On Twitter, fellow journalists implored users not to view the graphic video of the execution, instead sharing images of Foley in the field.

Dick Costolo, the CEO of the social media network, tweeted: "We have been and are actively suspending accounts as we discover them related to this graphic imagery."

Thousands posted messages of sorrow on the Facebook site Free James Foley, while British Prime Minister David Cameron tweeted that if true, the murder was "shocking and depraved."

'Finally found his passion'

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.

He came to journalism as a second career, enrolling in the Medill School of Journalism at Northwestern University aged 35. He had previously instructed inner-city students and taught reading and writing to prison inmates.

"He realised that the stories he wanted to tell were real stories - stories about people's lives -and he saw journalism as a vehicle for talking about what's really happening in the world," Diane Foley said in an earlier interview with the Columbia Journalism Review.

His father, John, told the same publication that before his ill-fated trip to Syria "Jim said that he finally had found his passion."

AFP chairman Emmanuel Hoog hailed Foley as a "brave, independent and impartial journalist."

"His work for AFP and other media organizations was widely admired. Nothing could justify his incarceration or any threats against his life. Our thoughts are with his family at this very difficult time."

GlobalPost co-founder and CEO Philip Balboni said: "On behalf of John and Diane Foley, and also GlobalPost, we deeply appreciate all of the messages of sympathy and support that have poured in since the news of Jim's possible execution first broke."

According to witnesses, Foley was seized in the northern Syrian province of Idlib on November 22, 2012.

Sotloff, whose kidnapping in August last year has not been widely reported, has written for several US newspapers and magazines, including Time, Foreign Policy and The Christian Science Monitor.

The White House said US intelligence was studying the video.

"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.

In the nearly five-minute video, titled "A Message to America" and distributed online by known Islamic State sources, the group declares that Foley was killed after Obama ordered air strikes against IS positions in northern Iraq.

The execution is carried out in an open desert area with no immediate signs as to whether it is in Iraq or Syria by a black-clad masked militant who speaks English with a British accent.