The parents of the executed US reporter James Foley said Friday the apparent death in an air strike of the Islamic State militant "Jihadi John" was of little solace to them.

The US military said it was "reasonably certain" the British extremist, who appeared in a string of graphic execution videos including Foley's, was killed in the joint British-US operation Thursday in Syria.

"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, whose son, a freelance journalist, was captured in Syria in 2012 and beheaded in August 2014.

"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 Steven Sotloff was killed in the same manner, again on camera and by Jihadi John.

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

"Hoping my brother is in heaven knowing some justice was served. Unfortunately this still doesn't change things," she wrote.

Britain's Prime Minister David Cameron said the death of Jihadi John, whose real name is Mohammed Emwazi, was "not yet certain."

A London computer programmer, Emwazi 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 Foley's beheading.

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.

'Jihadi John': quiet football fan who became IS symbol
London (AFP) Nov 13, 2015 - From a quiet, football loving child to an Islamic State executioner, the man who became one of the most haunting figures of the jihadist movement remains a mystery even after being the high-profile target of a US air strike.

Born Mohammed Emwazi, the masked 27-year-old Brit who became known as "Jihadi John" sparked worldwide revulsion with his grisly executions of foreign aid workers and journalists in Syria on camera.

People who knew him quoted by British media said they could not reconcile the quiet but intense young man they knew with the "cold, sadistic and merciless" killer described by one former hostage.

"Why did he do that? They say he's a very intelligent boy. How did he turn evil?" said James, 47, a former neighbour who gave only his first name.

James thought Emwazi was a "strange" boy, and would often see him cycling around their modest west London red-brick apartment block dressed in traditional Islamic attire.

Emwazi was born in Kuwait, but the family moved to London when he was six years old and he grew up in North Kensington, a leafy, middle-class area where a network of Islamist extremists has since been uncovered.

As a child he was a fan of Manchester United football club and the pop band S Club 7, according to a 1996 school year book published by The Sun tabloid.

"What I want to be when I grow up is a footballer," he wrote in the book.

He went on to study information technology at the University of Westminster, which confirmed that someone by that name left six years ago and said it was "shocked and sickened" by the allegations.

- 'Strange and unfriendly' -

The campaign group Cage, which published years of correspondence with Emwazi, blamed his radicalisation on a post-graduation trip to Tanzania in 2009.

Emwazi told Cage the trip was a holiday, but said he was accused by British authorities of planning to join Al-Shebab fighters in Somalia.

Following overnight detention at gunpoint in Dar es Salaam, the Tanzanian capital, Emwazi said he and his friends were sent back to Britain via Amsterdam, being interrogated in both ports, according to the correspondence released by the London-based charity.

He claimed that British intelligence services had been behind his detention, that they had asked him to become a spy and that they had promised him "a lot of trouble" after he rejected the offer.

On the advice of his mother and taxi-driver father, Emwazi flew to Kuwait to live with his fiancee's family and took up a job in IT, Cage said.

It was while trying to return to Kuwait after a visit to Britain in July 2010 that he claimed in his emails to Cage that authorities blocked him from travelling and put him on a terror watch list.

- 'Adrenaline junky' -

Court papers published by British media connected him to a network of extremists known as "The London Boys" -- originally trained by Al-Shebab.

The Guardian newspaper said some of them played football together.

The papers also linked him to Bilal al-Berjawi, who became a senior leader of Al-Shebab but was killed in a US drone attack in January 2012.

After changing his name to Mohammed al-Ayan and one final failed attempt to enter Kuwait in early 2013, he went missing, the Cage emails said.

Cage said the police told his family they believe he travelled to Syria after that.

How he rose to become one of the world's most wanted men is a mystery, but one hostage who fell under his control in the IS group's hub in Raqa talked of a "cold, sadistic and merciless" killer.

Two British trainee medics who met Emwazi when he visited friends in a Syrian hospital described him as "quiet, but a bit of an adrenaline junkie".

"I spotted this guy walking in, dressed in full combat kit, with a pistol on a holster, magazine, shopping bag in one hand and talking on a phone in the other," one of the medics told ITV News.

"He would bring drinks, sweets and ice cream".

They described hearing of one incident in which Emwazi drew his gun against a group of armed men who threatened to steal his weapons.

"He seems like someone with not a lot to lose," said the medic.