WASHINGTON: The US military conducted an air strike in Syria on Thursday targeting "Jihadi John", the masked, British-accented self-styled Islamic State (IS) militant seen in videos executing hostages, the Pentagon said.

Spokesman Peter Cook did not specify whether Mohammed Emwazi had been killed, saying in a statement that "we are assessing the results of tonight's operation and will provide additional information as and where appropriate." The Pentagon said the air strike took place in Raqa.

"Emwazi, a British citizen, participated in the videos showing the murders of US journalists Steven Sotloff and James Foley, US aid worker Abdul-Rahman Kassig, British aid workers David Haines and Alan Henning, Japanese journalist Kenji Goto, and a number of other hostages," the Pentagon said.

Word of the US action comes as Iraqi Kurdish forces backed by US-led air strikes blocked a key IS group supply line with Syria in the battle to retake the town of Sinjar from the jihadists.

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A permanent cut in the supply line would hamper IS's ability to move fighters and supplies between northern Iraq and Syria, where the jihadists hold significant territory and have declared a “caliphate".

Britain 'not yet certain'

British Prime Minister David Cameron said on Friday it was not yet clear whether the IS group's militant known as “Jihadi John” had been killed in a US air strike in Syria.

Cameron said the strike had targeted British citizen Mohammed Emwazi but added: “We cannot yet be certain if the strike was successful” in a statement delivered outside his Downing Street office.

He added: “This was an act of self-defence, it was the right thing to do."

Emwazi, a London computer programmer, was born in Kuwait to a stateless family of Iraqi origin.

His parents moved to Britain in 1993 after their hopes of obtaining Kuwaiti citizenship were quashed.

Dubbed "Jihadi John", he first appeared in a video in August of 2014 showing the beheading of Foley, a 40-year-old freelance journalist who had been missing since he was seized in Syria in November 2012.

Video of the beheading, titled "A Message to America", sparked worldwide revulsion. In it, IS declares that Foley was killed because President Barack Obama ordered air strikes against the group in northern Iraq.

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Foley is seen kneeling on the ground, dressed in an orange outfit that resembles those worn by prisoners held at the US naval base at Guantanamo Bay.

Two weeks later, his fellow US hostage Steven Sotloff was killed in the same manner, again on camera and by the same British-accented executioner.

On November 16, IS said it had executed Peter Kassig, a 26-year-old US aid worker kidnapped in Syria in October 2013, as a warning to Washington.

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