The United States targeted British Islamic State (IS) leader "Jihadi John" in an air strike in northern Syria and evidence was growing that he was killed.
A US official said Thursday's attack in the town of Raqqa probably killed Mohammed Emwazi, a British citizen who was nicknamed "Jihadi John" after appearing in videos showing the killings of American and British hostages.
The Syrian Observatory for Human Rights, a monitoring group, said four foreign militants had been killed in US air strikes.
"A car carrying four foreign IS leaders, including one British Jihadi was hit by US air strikes right after the governorate building in Raqqa city," Rami Abdulrahman, Director of the UK-based Observatory told Reuters.
"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."
The Pentagon said it was still assessing the effectiveness of the strike in Raqqa, the de facto capital of IS.
Emwazi 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 British government said it had "been working hand in glove with the Americans" to defeat Islamic State "and to hunt down those murdering Western hostages".
It said Prime Minister David Cameron had said "tracking down these brutal murderers was a top priority".
Describing Emwazi as a threat to the world, Cameron said in a televised statement that there was no certainty yet that the strike had been successful but that if it was it would be a strike at the heart of IS.
It would come more than a year after US President Barack Obama promised justice after the deaths of American hostages.
Dressed entirely in black, a balaclava covering all but his eyes and the bridge of his nose, Jihadi John became a menacing symbol of IS brutality and one of the world's most wanted men in videos showing the killing of hostages.
The strike came just as the United States seeks to increase pressure on IS fighters, who have seized parts of Syria and Iraq, and who Obama has vowed to defeat.
The pressure includes US plans to deploy dozens of special operations forces to Syria, deliver more weaponry to US-backed Syrian fighters and to thicken US air strikes against the militant group.