REPUBLIKA.CO.ID, BEIRUT - Islamic State militants said in a video on Sunday they had beheaded American hostage Peter Kassig and warned the United States they would kill other US citizens "on your streets".

The announcement of Kassig's death, in what would be the fifth such killing of a Western captive by the group, formed part of a 15-minute video posted online in which Islamic State showed the beheadings of at least 14 men it said were pilots and officers loyal to Syrian President Bashar al-Assad.

In Washington, President Barack Obama's National Security Council (NSC) said the US government was working to confirm the authenticity of the claim.

"If confirmed, we are appalled by the brutal murder of an innocent American aid worker and we express our deepest condolences to his family and friends," NSC spokeswoman Bernadette Meehan said in a statement.Other Western leaders and officials also condemned the killing.

A US government source said that because the latest video was more complex than earlier videos, it was expected to take longer than in the past for US agencies to make a definitive analysis. The analysis was still continuing, the source said.

The video did not show Kassig's beheading but showed a masked man standing with a decapitated head covered in blood lying at his feet. Speaking in English in a British accent, the man says: "This is Peter Edward Kassig, a US citizen."

Reuters could not immediately verify the authenticity of the footage, which appeared on a jihadist website and on Twitter feeds used by Islamic State.

In a statement, Kassig's parents Ed and Paula Kassig said they were aware of news reports "about our treasured son" and were awaiting confirmation as to their authenticity. They asked that in the meantime their privacy be respected.

The man in the video spoke with the same southern British accent as the killer of previous hostages, dubbed "Jihadi John" by British media. He was believed to have been wounded in an air attack on an IS leaders' meeting in Iraq near the Syrian border earlier this month, some media reports have said.

"To Obama, the dog of Rome, today we are slaughtering the soldiers of Bashar and tomorrow we will be slaughtering your soldiers," a masked militant says, predicting Washington would send more troops to the region to fight Islamic State.

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