BEIRUT (CBSNewYork/AP) — Islamic State extremists released a video posted Tuesday purportedly showing the beheading of a second American journalist, Steven Sotloff, and warning President Barack Obama that as long as U.S. airstrikes against the militant group continue, “our knife will continue to strike the necks of your people.”

In the wake of the beheading, and the growing threat from ISIS, President Obama announced Tuesday night that the U.S. is adding 350 more troops to help protect the American Embassy in Baghdad and its support facilities in the capital, raising the number of U.S. forces in the country to over 1,000.

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“If the video is genuine, we are sickened by this brutal act taking the life of another innocent American citizen,” said U.S. State Department spokeswoman Jen Psaki.

As CBS 2’s Dick Brennan reported, he footage was posted two weeks after the release of video showing the killing of James Foley and days after Sotloff’s mother pleaded for his life.

“I ask you to please release my child,” said Shirley Sotloff. “As a mother, I ask your justice to be merciful and not punish my son for matters he has no control over.”

Barak Barfi, a spokesman for the Sotloff family, said that the family had seen the video but that authorities have not established its authenticity.

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“The family knows of this horrific tragedy and is grieving privately. There will be no public comment from the family during this difficult time,” Barfi said.

Sotloff, 31, who freelanced for Time and Foreign Policy magazines, vanished in Syria in August 2013 and was not seen again until he appeared in a video released online last month that showed Foley’s beheading. Dressed in an orange jumpsuit against the backdrop of an arid Syrian landscape, Sotloff was threatened in that video with death unless the U.S. stopped airstrikes on the group in Iraq

In the video distributed Tuesday and titled “A Second Message to America,” Sotloff appears in a similar jumpsuit before he is apparently beheaded by a fighter with the Islamic State, the extremist group that has claimed wide swathes of territory across Syria and Iraq and declared itself a caliphate.

The SITE Intelligence Group, a U.S. terrorism watchdog, first reported about the video’s existence. Unlike Foley’s beheading, which was widely shared on Twitter accounts affiliated with the Islamic State group, the video purporting to show Sotloff’s killing was not immediately posted online, though several jihadi websites told users to expect it Tuesday.

As of late Tuesday afternoon, U.S. officials were carefully going over the videotapes, in hopes of identifying the executioner — who appears to speak with a British accent.

“The tape is pure propaganda,” said CBS News Military Analyst Major Mike Lyons. “It’s actually for recruiting other jihadists; recruiting people to come and work on ISIS; it’s for their cause. It’s not anything for, really, American consumption.

Before the video’s release, messages on websites frequented by jihadis warned of a “second message to America.” However, it appeared that a separate faction of the Islamic State group posted it early to another account before it was supposed to be released. A later Twitter message apologized for releasing it early and asked fellow jihadis not to “reproach” them.

White House press secretary Josh Earnest said he wasn’t immediately aware of the purported Sotloff video and wasn’t in a position to confirm its authenticity.

“This is something that the administration has obviously been watching very carefully,” Earnest said. “Our thoughts and prayers first and foremost are with Mr. Sotloff and Mr. Sotloff’s family and those who worked with him.”

A man who answered a phone listed in the name of Sotloff’s sister hung up when called by the AP.

In the wake of the beheading, and the growing threat from ISIS, President Obama announced Tuesday night that the U.S. is adding 350 more troops to help protect the American Embassy in Baghdad and its support facilities in the capital, raising the number of U.S. forces in the country to over 1,000.

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Meanwhile, the White House said in a statement Tuesday night that President Obama has approved the additional troops for protection of American personnel following a request by the State Department and a review and recommendation by the Defense Department.

The additional troops will not serve in a combat role, the White House said. Most are from the Army and some are Marines, the Pentagon said in a statement.

Approximately 820 troops have now been assigned to augment diplomatic security in Iraq, said Rear Adm. John Kirby, the Pentagon’s spokesman.

The additional troops will come from within the U.S. Central Command area of operations and will include a headquarters element, medical personnel, associated helicopters and an air liaison team, Kirby said. Fifty-five troops in Baghdad since June will be redeployed outside of Iraq and replaced by 405 newly deployed troops, he said.

Long Island U.S. Rep. Peter King (R-N.Y.), who called the Foley beheading an act of war, said before the announcement that the U.S. needs to take stronger action against ISIS — both in Iraq and Syria — and arm those whose land the Islamic State is trying to take over, 1010 WINS’ Sonia Rincon reported.

“What this shows is ISIS is a ruthless, brutal jihadist organization,” King told 1010 WINS. “It’s a terrorist gang and the United States has to stop it. What they’ve done in these two beheadings is just a very small example of the type of atrocities that they want to carry out throughout the world.”

The fighter who beheads Sotloff in the video called it retribution for Obama’s continued airstrikes against the group in Iraq.

“I’m back, Obama, and I’m back because of your arrogant foreign policy towards the Islamic State — despite our serious warnings,” the fighter said. “So just as your 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 Cawthorne Haines. It was not immediately clear who Haines was. Officials with the British Foreign Office declined to immediately comment.

“ISIS has a large group of American hostages, including an American woman, who could possibly be beheaded in a similar tactic that James Foley was, and this individual was as well,” Lyons said.

Psaki said it’s believed that “a few” Americans are believed to still be held by the Islamic State but would not give any specifics.

President Obama heads to a NATO summit later this week, where he will work to enlist more international support for pushing back ISIS in Iraq, and possibly targeting the group’s stronghold in Syria.

“The world has to, and with the United States leading, has to make sure that ISIS isn’t allowed to plot and plan attacks against our homeland, and our NATO allies need to be involved too,” said U.S. Rep. Eliot Engel (D-N.Y.), who sits on the House Foreign Affairs Committee.

Republicans have criticized President Obama for saying he does not have a strategy for ISIS in Syria. The State Department promised a firm U.S. response to what happened.

The Islamic State group has terrorized rivals and civilians alike with widely publicized brutality as it seeks to expand a proto-state it has carved out on both sides of the border.

In its rise to prominence over the past year, the extremist group has frequently published graphic photos and gruesome videos of everything from bombings and beheadings to mass killings.

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