A drone strike targeting the British IS militant known as "Jihadi John" has been carried out in Syria - and the US is "99% sure we got him".
Activists in the IS stronghold Raqqa said they counted 14 airstrikes between 11.51pm and midnight on Thursday.
They reported one of the first missiles targeted a car near the city's Islamic court - but the area was closed off by militants to prevent anyone from approaching.
Eyewitnesses in Raqqa told Sky News Jihadi John - real name Mohammed Emwazi - was taken to hospital after the strike, with Islamic State claiming he was injured but still alive.
Although the official line from IS leaders is that Emwazi survived, locals told Sky the hospital has since been closed to the public and claimed this only happens when a senior militant has been killed.
US Secretary of State John Kerry said: "We are still assessing the results of this strike but the terrorists associated with Daesh (another name for IS) need to know this - your days are numbered and you will be defeated."
However, a senior US defence official told Fox News: "We are 99% sure we got him. We were on him for some time."
Another source told ABC News Emwazi was blown up in a "flawless" and "clean hit".
A Pentagon spokesman said later it was "reasonably certain" he had been killed by a Hellfire missile from a drone.
Describing Emwazi as a human animal, Colonel Steve Warren said he was "the only high-value individual" targeted in the strike.
He also revealed the operation was the latest in a series of attacks on IS leaders. He said the US had killed one mid- to upper-level IS member every two days since May.
David Cameron said the hunt for the IS executioner was a "combined effort" with the US and singling him out was an act of self-defence and "the right thing to do".
British experts contributed by helping with intelligence gathering to "build the picture" ahead of the airstrike, Sky's Defence Correspondent Alistair Bunkall said.
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UK-based monitoring group the Syrian Observatory for Human Rights said three other foreign IS fighters may have been killed with Emwazi.
The group's director Rami Abdulrahman said: "A car carrying four foreign Islamic State leaders, including one British Jihadi was hit by US airstrikes right after the governorate building in Raqqa city.
"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."
Jihadi John was first seen, his face covered, in a video in August 2014 which showed the beheading of US journalist James Foley.
The extremist appeared in a number of other beheading videos, including those in which UK hostages Alan Henning and David Haines were killed.
He also appeared in videos showing the killings of American journalist Steven Sotloff, aid worker Abdul-Rahman Kassig and Japanese journalist Kenji Goto.
David Haines' daughter Bethany said: "After seeing the news that 'Jihadi John' was killed I felt an instant sense of relief, knowing he wouldn't appear in anymore horrific videos."
John and Diane Foley, the parents of James Foley said in a statement Emwazi's death would be "really a small solace to us".
Alan Henning's nephew Stuart Henning tweeted: "Mixed feelings today wanted the coward behind the mask to suffer the way Alan and his friends did but also glad it's been destroyed."
Emwazi was born in Kuwait and moved to the UK with his family when was six. He attended state schools, then studied computer science at the University of Westminster before leaving for Syria in 2013.
He and three other IS militants with British accents - whose identities have not been made public - were nicknamed "the Beatles" by some of their captives.
The airstrike has again highlighted the controversial issue of extrajudicial targeting of British citizens overseas.
Campaign group CAGE and Labour leader Jeremy Corbyn both said Emwazi should have been captured and put on trial.
Elsewhere, officials in Turkey said a suspected British associate of Emwazi has been detained in Istanbul.
He is thought to be Aine Lesley Davis, one of a group of UK Islamists who apparently guarded foreign prisoners in Syria.