The US military conducted an air strike in Syria targeting "Jihadi John," the masked Islamic State (IS) militant with a British accent seen in grisly videos executing Western hostages, the Pentagon said.
Spokesman Peter Cook did not specify whether Mohammed Emwazi had been killed, saying 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 Thursday air strike took place in Raqa, the Islamic State group's 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.
In Britain, Prime Minister David Cameron's office said he would make a statement later on Friday.
"We have been working hand in glove with the Americans to defeat ISIL (another name for IS) and to hunt down those murdering hostages," a spokesman said.
"The prime minister has said before that tracking down these brutal murderers was a top priority."
The Syrian Observatory for Human Rights, a Britain-based monitoring group, said four people were killed in a strike in Raqa late on Thursday.
"The car was hit in the centre of town, near the municipality building," Observatory chief Rami Abdel Rahman said.
He said sources described one of those killed as a "senior British member of the group".
- Accused of multiple murders -
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 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.
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In it, IS declares that Foley was killed because President Barack Obama ordered air strikes against the group in northern Iraq.
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, 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.
Sotloff's mother, Shirley Sotloff, told NBC News following word of Thursday's strike that she had not been informed about it and that, even if Emwazi had been killed, "it doesn't bring my son back."
"Who knows if he's gone," NBC News quoted her as saying. "I don't think there will ever be closure."
On November 16, IS said it had executed Peter Kassig, a 26-year-old US aid worker kidnapped in Syria in October 2013, again as a warning to Washington.
- 'Cold, sadistic, merciless' -
"Jihadi John" 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 a fan of Manchester United football club and the band S Club 7, according to a 1996 school year book published by The Sun tabloid. He later went on to study information technology at the University of Westminster.
The university confirmed that someone by that name left six years ago, and said it was "shocked and sickened" by the allegations.
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.
One hostage who fell under Emwazi's control in the IS group's hub in Raqa talked of a "cold, sadistic and merciless" killer.
Word of the US action comes as Iraqi Kurdish forces, backed by US-led air strikes and ground spotters, blocked a key IS supply line with Syria in the battle to retake the town of Sinjar from the jihadists.
Early on Friday, Kurdish forces entered the town, where IS carried out a brutal campaign of killings, enslavement and rape against the Yazidi minority last year, an AFP correspondent reported.
Aiding the Yazidis, whose unique faith Sunni Muslim group IS considers heretical, was one of Washington's main justifications for starting its air campaign against the jihadists last year.