WASHINGTON (AFP/REUTERS) - A masked Islamic State in Iraq and Syria (ISIS) militant nicknamed "Jihadi John" has been killed, the BBC reported, after the United States military conducted an air strike in Syria on Thursday (Nov 12) targeting him.

In a Twitter post on Friday (Nov 13), the BBC said that "Jihadi John" had been killed in the US air strike, quoting a senior military source, who had a "high degree of certainty".

The militant with a British accent was often seen in grisly videos executing Western hostages, the Pentagon said.

Monitoring group the Syrian Observatory for Human Rights also said on Friday that a "British" leader of ISIS and three other foreign militants have been killed in the US-led air strikes in the northern Syrian town of Raqqa.

"A car carrying four foreign Islamic State leaders, including one British jihadi, was hit by US air strikes right after the governorate building in Raqqa city," Mr Rami Abdulrahman, director of the Britsh-based Observatory, told Reuters.

"All the sources there are saying that the body of an important British jihadi is lying in the hospital of Raqqa. All the sources are saying it is of Jihadi John but I cannot confirm it personally."

The Observatory did not say whether the British national was the militant known as "Jihadi John", after a US official said early on Friday that a strike in the area probably killed him.

Spokesman Peter Cook said 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 Raqqa, ISIS' de facto Syrian capital.

"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.

CNN and the Washington Post, citing officials, reported that Emwazi was targeted by a drone.

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

Kurdish peshmerga forces also secured several strategic facilities in the northern Iraqi town of Sinjar on Friday as part of an offensive against ISIS militants that could provide critical momentum in efforts to defeat the group.

"ISIL defeated and on the run," the Kurdistan regional security council said in a tweet, using an acronym for ISIS.

It said the peshmerga had secured Sinjar silo, cement factory, hospital and several other public buildings.A permanent cut in the supply line would hamper ISIS' ability to move fighters and supplies between northern Iraq and Syria, where the militants hold significant territory and have declared a "caliphate".

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" by British and US media, he first appeared in a video in August of 2014 showing the beheading of Mr Foley, a 40-year-old American 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, ISIS declares that Mr Foley was killed because US President Barack Obama ordered air strikes against the group in northern Iraq.

Mr 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.

Emwazi is dressed entirely in black and wears a mask.

Two weeks later, Mr Foley's fellow US hostage Steven Sotloff was killed in the same manner, again on camera and by the same executioner with a British accent.

Mr Sotloff's mother Shirley Sotloff told NBC News following word of Thursday's strike that she hadn't been informed about it and that, even if Emwazi had been killed, "it doesn't bring my son back".

"I don't think there will ever be closure."

On Nov 16, 2014, ISIS said it had executed Mr Peter Kassig, a 26-year-old US aid worker kidnapped in Syria in October 2013, again as a warning to Washington.