Video purports to show beheading of reporter Steven Sotloff

BEIRUT (KABC) -- A new ISIS video posted online Tuesday appears to show the beheading of U.S. reporter Steven Sotloff by a self-professed member of ISIS.

In the video, entitled "A Second Message to America," Sotloff addresses the camera, saying, "I'm sure you know exactly who I am by now and why I am appearing."

Sotloff, a 31-year-old American journalist who has written for Time and Foreign Policy magazines, was last seen in Syria in August 2013 before he appeared last month in the video that showed the beheading of journalist James Foley.

Militants threatened to kill Sotloff next. His mother, Shirley Sotloff, then released a video pleading to the Islamic State group for his release.

"Steven has no control over the actions of the U.S. government. He's an innocent journalist," she said. "I've always learned that you, the caligh, can grant amnesty. I ask you to please release my child. As a mother, I ask your justice to be merciful and not punish my son for matters he has no control over."

ISIS ignored his mother's plea.

"Obama, your foreign policy of intervention in Iraq was supposed to be for preservation of American lives and interests, so why is it that I am paying the price of your interference with my life," Sotloff says in the new video.

The video cuts to the militant, who appears by Sotloff's side holding a knife. He says as long as U.S. missiles "continue to strike our people, our knife will continue to strike the necks of your people."

At the end of the video, he threatened to kill a third captive, a Briton, David Cathorne Haines. It was not immediately clear who Haines is.

White House spokesman Josh Earnest said that while the White House has not yet confirmed the authenticity of the video, "Our thoughts and prayers are with Mr. Sotloff and Mr. Sotloff's family and those who worked with him."

Earnest, speaking on behalf of President Barack Obama, said extensive efforts were made to rescue Sotloff.

"The United States, as you know, has dedicated significant time and resources to try and rescue Mr. Sotloff," he said.

Meantime, State Department spokeswoman Jen Psaki said U.S. intelligence analysis will "work as quickly as possible" to determine if the video is authentic.

"If the video is genuine, we are sickened by this brutal act, taking the life of another innocent American citizen," Psaki said.

The Committee to Protect Journalists said, "being butchered in front of the camera simply for being a reporter is pure barbarism."

Intelligence analysts say slaughter of journalists as well as Iraqis and Syrians play into ISIS militants' reign of terror and their crusade to spread extremist beliefs.

"When you get this narrow line, and anybody that's not on their side of the line is not human and deserves to die," ABC News consultant and former FBI agent Brad Garrett said.

USC professor Sandy Tolan is a veteran foreign correspondent.

"Clearly what they're trying to do is go into the sort of the terrible heart of Americans' fears," he said.

Tolan says that Sotloff was writing to explain a complex story, how U.S. involvement in the Gulf War forged new enemy factions and alliances now fighting back.

"There is a new force at work in the region that is far more extremist and far more brutal than anything that we understood up until very recently," Tolan said.

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In a statement, family spokesman Barak Barfi said Sotloff's family, "knows of this horrific tragedy and is grieving privately."

ABC News and The Associated Press contributed to this report.