Islamic State insurgents released a video on Tuesday purportedly showing the beheading of U.S. journalist James Foley, who had gone missing in Syria nearly two years ago, and images of another U.S. journalist whose life they said depended on U.S. action in Iraq.
The video, titled "A Message To America," was posted on social media websites. It was not immediately possible to verify its authenticity.
Foley, who has reported in the Middle East for five years, was kidnapped on Nov. 22, 2012, by unidentified gunmen. Steven Sotloff, who appeared at the end of the video, went missing in northern Syria while he was reporting in July 2013.
A Twitter account set up by Foley's family to help find him said early on Wednesday: "We know that many of you are looking for confirmation or answers. Please be patient until we all have more information, and keep the Foleys in your thoughts and prayers."
The White House said that U.S. intelligence agents were working to verify the authenticity of the video.
Islamic State, an al-Qaeda offshoot, had not previously executed American citizens publicly. The Sunni militant group has declared a caliphate in parts of Iraq and Syria in areas it controls.
The video, which was posted after the United States resumed air strikes in Iraq for the first time since the end of the U.S. occupation in 2011, opened with a clip of U.S. President Barack Obama saying he had authorised strikes in Iraq.
"Obama authorises military operations against the Islamic State effectively placing America upon a slippery slope towards a new war front against Muslims," words appear in English and Arabic on the screen.
It showed black and white aerial footage of air strikes with text saying "American aggression against the Islamic State"
A person identified as James Foley and wearing an orange outfit is seen kneeling in the desert as a man in black dress with a black mask stands beside him, holding a knife.
"I call on my friends family and loved ones to rise up against my real killers, the U.S. government, for what will happen to me is only a result of their complacency and criminality," the kneeling man says.
The man in the mask speaks in a British accent and says: "This is James Wright Foley, an American citizen, of your country. As a government, you have been at the forefront of the aggression towards the Islamic State."
"Today your military air force is attacking us daily