The Islamic State group has released a video purportedly showing one of its fighters beheading US journalist James Foley, who had gone missing in Syria nearly two years ago.
The video, titled "A Message To America," was posted on social media sites on Tuesday, suggesting that it was an act of revenge for US air strikes against the group's fighters in Iraq.
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.
Foley's mother, Diane Foley, released a statement through a Twitter account the family had used to campaign in which she confirmed his death and said he died "trying to expose the world to the suffering" of the Syrian people.
"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," the statement read.
"We thank Jim for all the joy he gave us. He was an extraordinary son, brother, journalist and person. Please respect our privacy in the days ahead as we mourn and cherish Jim."
The group claimed in the video to be holding another US journalist and said his life depended on US President Barack Obama's next move.
"The life of this American citizen, Obama, depends on your next decision," said a masked man in the video posted on social media sites, speaking English with a British accent as he held a prisoner the video named as Steven Sotloff.
The video could not immediately be verified.
Foley, 41, was contributing videos to the AFP news agency for the media company GlobalPost before he was kidnapped on November 22, 2012 by unidentified gunmen.
"We have seen a video that purports to be the murder of US citizen James Foley by ISIL. The intelligence community is working as quickly as possible to determine its authenticity," Caitlin Hayden, a spokesperson for the US National Security Council, said on Wednesday.
"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. We will provide more information when it is available."
The IS has an army of more than 50,000 fighters in Syria, and recruited 6,000 people in the last month, a monitoring group said on Tuesday.
Rami Abdel Rahman, the director of the UK-based Syrian Observatory for Human Rights, said that the group's recruitment push was gathering pace every month.
Abdel Rahman said the new recruits in Syria included more than 1,000 foreign fighters from Chechnya, China, Europe and Arab countries. He said most had entered Syria from Turkey.
The IS, formerly known as the Islamic State of Iraq and the Levant (ISIL), grew out of the US-led war in Iraq, and entered the civil war in Syria last year.
It has recently taken over vast areas of neighbouring Iraq and its leader, Abu Bakr al-Baghdadi, earlier this year declared himself the caliph of territory captured on both sides of the border.