The Islamic State has released a video of the purported murder of the US journalist Steven Sotloff, days after his mother pleaded with the group's leader to spare his life.

The armed group released the video of what it said was the 31-year-old's death on Tuesday. Internet monitors who have seen the video confirmed its content. Sotloff had been held hostage in Syria by the group since August 2013.

A US spokeswoman said Washington was trying to authenticate the "sickening" footage. British Prime Minister David Cameron said it depicted an "absolutely disgusting, despicable act."

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The death comes two weeks after the group released a video showing the murder of James Foley, another US journalist, which a masked member of the Islamic State claims was revenge for US attacks on its fighters in Iraq.

In that video, Sotloff was pictured, with the fighter saying his life depended on US President Barack Obama's next move.

The killer in it appears to be the same man in both videos - a British-accented man dressed in black fatigues and balaclava.

Internet monitors who have viewed both videos said that Sotloff's hair was longer in the second.

Less than a week ago, Sotloff's mother, Shirley, directly addressed the group's leader Abu Bakr al-Baghdadi in an appeal for his release. 

"You, the caliph, can grant amnesty. I ask you, please, release my child. I ask you to use your authority to spare his life,'' Mrs Sotloff said in the video.

More troops to Iraq

Jan Psaki, of the US state department, said authorities were working to authenticate the video.

"If true, we are sickened by this brutal act. Our hearts go out to the Sotloff family," she said.

Several westerners are being held hostage by rebel groups in Syria.

On August 24, al-Qaeda's Syrian affiliate the Nusra Front freed the American writer, Peter Theo Curtis, who had been missing since 2012, following what officials said were efforts by the Gulf state of Qatar.

Meanwhile, Obama has approved sending 350 more troops to help protect the US Embassy in Baghdad and its support facilities in the Iraqi capital, raising the number of US forces to more than 1,000, the White House said in a statement.

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.

The additional troops will include a headquarters element, medical personnel, associated helicopters and an air liaison team, said Rear Admiral John Kirby, the Pentagon's spokesman.