The US military said Friday it was "reasonably certain" that the Islamic State executioner "Jihadi John," a British citizen, was killed in a drone strike in Syria.

Mohammed Emwazi, whose masked figure appeared in a string of graphic videos showing the beheading of Western hostages, was targeted in a combined British-US operation Thursday in Raqa, the de facto IS capital in war-torn Syria.

In a briefing webcast from Baghdad to Pentagon reporters, Colonel Steve Warren said it would take time for formal confirmation that the Hellfire missile drone strike killed the notorious 27-year-old militant.

But Warren said the US had "great confidence that this individual was Jihadi John."

"We know for a fact that the weapons system hit its intended target, and that the personnel who were on the receiving end of that weapons system were in fact killed," he said. "We are reasonably certain that we killed the target that we intended to kill, which is 'Jihadi John'."

"This guy was a human animal, and killing him probably makes the world a little bit better place," Warren added.

Britain's Prime Minister David Cameron had earlier said Emwazi's death was "not yet certain," but that if confirmed, it would be "a strike at the heart" of the IS group.

Analysts said the impact of Emwazi's death would likely be symbolic rather than tactical for the jihadist group, which controls swathes of Iraq and Syria, where they have declared a "caliphate," and is known for widespread atrocities.

But the operation coincided with the launch of a major US-backed offensive in northern Iraq.

US Secretary of State John Kerry, speaking on a visit to Tunis, warned the group's "days are numbered," as Iraqi Kurdish officials said IS had been beaten back from the town of Sinjar.

"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 in a statement.

He was last seen in the video showing Goto's execution in January.

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" after hostages nicknamed a group of IS guards The Beatles, he first appeared in a video in August 2014 showing the beheading of Foley, a 40-year-old American freelance journalist captured in Syria in 2012.

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

The reporter's parents said Friday the executioner's death was of little consolation to them.

"It is a very small solace to learn that Jihadi John may have been killed by the US government," John and Diane Foley said in a statement.

"His death does not bring Jim back," said the couple. "If only so much effort had been given to finding and rescuing Jim and the other hostages who were subsequently murdered by ISIS, they might be alive today."

Two weeks after Foley, fellow US hostage Sotloff was killed in the same manner, again on camera and by the same executioner.

Sotloff's sister, Lauren, posted on Facebook that the militant "should of (sic) had his head cut off also and been left to suffer. But at least he is dead."

Bethany Haines, whose father David was killed, told ITV News: "After seeing the news that 'Jihadi John' was killed I felt an instant sense of relief."

Raffaello Pantucci of the Royal United Services Institute think tank in London said Emwazi's death would make little strategic difference and could create a "martyr culture" around him.

But Charlie Winter, an academic who focuses on IS activities, said it could be a "big blow."

Emwazi was six years old when his family moved to London. He grew up in North Kensington, a leafy middle-class area where a network of Islamist extremists was uncovered in recent years.

As a child, he was said to be a fan of Manchester United football club and the S Club 7 pop group. He later went on to study information technology at the University of Westminster.

Court papers published by British media connected Emwazi to a network of extremists known as "The London Boys" that were originally trained by the Shebab, Al Qaeda's East Africa affiliate.

Cage, a London human rights group which worked with Emwazi in Britain before he left for Syria, reacted by saying he should have faced trial, not death.

"Emwazi should have been tried as a war criminal," the group said in a statement.