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ISIS video shows execution of US journalist Steven Sotloff

The militant group the Islamic State of Iraq and Syria has beheaded American journalist Steven Sotloff, the US confirmed Wednesday.

A second American journalist was  beheaded by the Islamic State of Iraq and Syria (ISIS) in a video made public Tuesday.

The video shows the death of Steven Joel Sotloff, 31, by what appears to be the same masked jihadist who killed fellow journalist James Foley, 40, exactly two weeks ago.

The U.S. confirmed the authenticity of the video early Wednesday. 

Sotloff, a Florida-based freelance journalist, went missing in northern Syria over a year ago. He appeared at the end of the ISIS video showing Foley's execution, which warned that Sotloff would be the next to die if the U.S. continued airstrikes against the militant group in Iraq.

Sotloff's execution, despite pleas from his family and promises from the United States to protect American lives in the region, has intensified calls for President Obama to expand the U.S. military campaign against ISIS to include airstrikes in Syria, where the group has its headquarters. Late on Tuesday, the president did authorize the deployment of approximately 350 additional U.S. soldiers to Iraq, but not "in a combat role."

The video opens with a clip of Obama speaking two weeks ago in the wake of Foley's beheading. During the address, the president pledged "relentless" commitment to protecting American citizens and bringing ISIS to justice. The video then shows Sotloff in an orange jumpsuit, kneeling beside a masked militant with a knife at his side. Sotloff makes a statement saying he is paying the price of U.S. airstrikes in Iraq. The masked man then addresses the camera before beheading Sotloff. Later in the video, another man, identified as a British citizen, appears in an orange jumpsuit alongside the fighter in an apparent threat to prisoner’s life.

A colleague of the man shown at the end of the video told that the prisoner is British and an NGO worker.

RELATED: Letter from journalist James Foley details detention

British Prime Minister David Cameron condemned the video as “disgusting and despicable,” according to the UK's Press Association. He said he would follow up with a full statement later.

Sotloff's family is aware of the video and is grieving, a spokesman told NBC News. Last week, Sotloff's mother, Shirley Sotloff, released a video begging ISIS to spare her son's life.

"Our thoughts and prayers, first and foremost, are with Mr. Sotloff and Mr. Sotloff's family and those who worked with him," White House press secretary Josh Earnest said Tuesday, adding that he couldn't confirm the authenticity of the video that purportedly shows the execution.

State Department spokesperson Jen Psaki said the U.S. is aware of reports of the video. She said U.S. intelligence officials would work to determine its authenticity.

"If the video is genuine, we are sickened by this brutal act," Psaki said. 

The reported release of Sotloff's execution video follows a similar one of Foley's. In the video showing Foley's beheading, the Islamic militant group threatened Sotloff's life unless the U.S. halted airstrikes in Iraq.

Sotloff was a freelance journalist who worked with Time and Foreign Policy magazines. He was seized in Syria in August 2013 and had not been seen since the video of Foley’s execution. 

“We are shocked and deeply saddened by reports of Steven Sotloff's death," Time editor Nancy Gibbs said in a statement. "Steven was a valued contributor to TIME and other news organizations, and he gave his life so readers would have access to information from some of the most dangerous places in the world. Our thoughts and prayers are with him and his family.”

In another statement, the Council on American-Islamic Relations (CAIR), the nation's largest Muslim civil rights and advocacy organization, condemned the killing and called for “redoubled efforts by people of all faiths and backgrounds to promote peace and justice.”

Florida Democratic Sen. Bill Nelson took a more aggressive stance, saying, “we must go after ISIS right away because the U.S. is the only one that can put together a coalition to stop this group that’s intent on barbaric cruelty.” He later pledged to introduce legislation giving President Obama explicit authority to order airstrikes against ISIS in Syria, according to NBC News’ Frank Thorp.

California Rep. Ed Royce, chairman of the House Foreign Affairs Committee, also called for the U.S. to work with allies in “targeting ISIS from the the air with drone strikes” and arming “the Kurds on the ground who are fighting them.”

But arguably the most blunt reaction came from Illinois Sen. Mark Kirk, who was asked about the ISIS beheading at a veterans event in Chicago. “We should bomb the hell out of them,” said the Republican lawmaker.

Obama pushed back against media reports of planned military strikes against ISIS in Syria at a press conference last week, stating that the U.S. “[doesn’t] have a strategy yet.” Obama has yet to comment on the Sotloff execution video, but some Republicans took the opportunity to demand action from the White House.

“I think I can speak for all Floridians and all Americans when I say that the time for a strategy is now, and part of that strategy needs to include destroying them,” said Florida Gov. Rick Scott in a statement. Republican Sen. Lindsey Graham took a similar tack.

"Mr. President, if you can't come up with a strategy, at least tell us what the goal is regarding ISIL," said Graham in a statement. 

Louisiana Gov. Bobby Jindal, a potential 2016 candidate, lent his voice to the fray as well, accusing President Obama of lacking a foreign policy altogether.

"I have no doubt about the president's sorrow over the two murders of American citizens by ISIS,” said Jindal in a statement. “I am certain that this grieves him deeply. And while grieving is important, it is no substitute for a strategy.”

RELATED: Republican lawmakers urge aggressive response to ISIS

For more than two years, the Committee to Protect Journalists (CPJ) has ranked Syria as the most dangerous place in the world to be a member of the press. Though journalists who cover the war are aware of that risk, said CPJ Executive Director Joel Simon, “being butchered in front of a camera simply for being a reporter is pure barbarism.”

“We condemn in the strongest terms possible the murder of journalist Steven Sotloff,” Simon said in a statement. “He, like James Foley, went to Syria to tell a story. They were civilians, not representatives of any government. Their murders are war crimes and those who committed them must be brought to justice swiftly.”

This is a developing story. Check back for more updates.