UPDATE (11:45 a.m. EDT): British officials say the militant in the video with American journalist James Foley appears to be British. Although the masked man who killed Foley never shows his face, he speaks fluent English in what linguists say sounds like a London accent.

British Foreign Secretary Philip Hammond said his government is aware that some of its citizens have been involved in committing atrocities perpetrated by ISIS and other extreme groups. Several hundred British nationals have traveled to Syria to join rebels fighting President Bashar Assad and likely have crossed into Iraq to fight with ISIS, officials say.

Their involvement in terror operations has increased fears they might come home and try to carry out attacks on British soil.

UPDATE (9:15 a.m. EDT, June 20): U.S. officials have confirmed the authenticity of a video that shows ISIS militants beheading American journalist James Foley. In a statement posted on Facebook, his family said they had never been more proud of him.

"He gave his life trying to expose the world to the suffering of the Syrian people," said the statement, which was attributed to Foley's mother, Diane Foley. She implored the militants to spare the lives of other hostages. "Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world."

The militants have threatened to kill another American journalist if the United States continues airstrikes against ISIS fighters in Iraq.

Associated Press/Photo by Steven Senne, File Journalist James Foley poses for a photo during a 2011 interview with The Associated Press, in Boston.

OUR EARLIER REPORT (10:45 p.m. EDT, June 19): U.S. intelligence officials are trying to verify the authenticity of a video released Tuesday by Islamic militants in Syria that purportedly shows an American journalist's beheading.

In the video, journalist James Foley, missing for two years, reads a statement in which he blames his death on America. He also said he wished he had more time to see his family again. The militant standing behind him then kills him with a sword.

At the end of the video, the militants show another man, identified as American journalist Steven Sotloff, and warn he will be killed if the United States does not call off strikes against ISIS fighters in Iraq.

Foley, who worked for Agence France-Press and the Boston-based media company GlobalPost, went missing in Syria in 2012. Sotloff was kidnapped near the Syrian-Turkish border in 2013. He freelanced for Time, the National Interest, and MediaLine.

Foley's family and employers said they were waiting on confirmation of his death before making any statement. "We ask for your prayers for Jim and his family," said Philip Balboni, GlobalPost CEO and co-founder.

ISIS militants recently threatened to kill Foley over the airstrikes against their fighters around Mount Sinjar, the Mosul Dam, and the Kurdish capital of Erbil. The U.S. strikes provided cover for the Kurdish peshmerga forces to retake ground and get the upper hand on ISIS fighters, who up until that point had overrun Iraqi forces.

The militants, which also go by the name Islamic State, now control a swath of land that stretches from Syria to Iraq. Their goal is to establish an Islamic Caliphate in the region. They are known for their brutality against anyone who doesn't share their brand of extremist Islam. even al-Qaeda has disavowed their tactics.

The Associated Pres contributed to this report.