The US military has conducted an air strike in Syria targeting "Jihadi John," the masked Islamic State militant with a British accent seen in grisly videos executing Western hostages.
Pentagon 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 (local time) air strike took place in Raqa, the Islamic State (IS) 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".
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.
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".