The world's most wanted man, the Islamic State (ISIS) executioner known as "Jihadi John," was killed in a U.S. drone strike on Thursday night as he left a building in Raqqa, Syria, "evaporating" in a "flawless clean hit" while about to board a vehicle, U.S. officials announced.

"U.S. forces conducted an airstrike in Raqqa, Syria, on Nov. 12, 2015 targeting Mohamed Emwazi, also known as 'Jihadi John,'" Pentagon Press Secretary Peter Cook said.

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

Pentagon officials said they are "99 percent" sure they have assassinated Emwazi, the 27-year-old Londoner who became the face of ISIS, the MailOnline reported.

It was the kind of death that the notorious killer had feared the most. In September the MailOnline reported that Emwazi had a "crippling fear of being killed in a drone attack and shielded himself behind civilians in the hope it will stop him being targeted."

The British news outlet said it got this information after two visitors to ISIS territory described how Emwazi and his cohorts once "fled onto a football pitch packed with civilians in a desperate attempt to dodge a drone attack."

The killing of Emwazi would be a major blow to ISIS, according to Richard Clarke, a former counter-terrorism advisor to the White House and current ABC News consultant.

"Since ISIS has used propaganda and its 'winner' image to lure new adherents, when its propaganda figure is killed that makes it look more like a loser, more like the tide may be turning against it," Clarke told ABC News.

"Jihadi John" was unmasked and identified as Kuwaiti-born Emwazi in media reports in February. Since then he had ceased to appear in ISIS videos.

Emwazi was at the top of the British government's "kill list" of up to a dozen British radicals. He has been the subject of a manhunt for more than a year, after he first appeared in a beheading video in August 2014, according to MailOnline.

Emwazi lived in Britain since the age of six. He had been known to British intelligence services but managed to travel to Syria in 2013, sources said.

In his first execution video that was leaked to Western media, the tall masked "Jihadi John" was shown clad in black and speaking in a British accent.

He began the execution of the kneeling Foley with a political rant. The video ended with him holding a large knife in his hand and his headless victim lying before him in the sand.

Diane Foley, the mother of Emwazi's first victim, told ABC News that Emwazi's would be "really a small solace to us."

She wondered why the authorities could not "make half that effort [launching an airstrike on Emwazi] to save the hostages while these young Americans were still alive."

A senior US defence official told CNN that they knew it was Emwazi before they struck in a mission he described as involving "persistent surveillance."

Shirley Sotloff, the mother of another of Emwazi's victim, said the ISIS butcher's death would not bring closure.

"If they got him, great. But it doesn't bring my son back," she told NBC News.