The Pentagon says it is "reasonably certain" that a US drone strike in Syria has killed the Islamic State militant known as "Jihadi John".
Col Steve Warren said Thursday's attack in Raqqa had hit the intended target, but it would take time to "formally declare that we have had success".
Mohammed Emwazi, a Kuwaiti-born British militant, appeared in videos of the beheadings of Western hostages.
UK PM David Cameron said targeting Emwazi was "the right thing to do".
It is believed there was one other person in the vehicle targeted in the strike.
Col Warren said such "routine" attacks had killed on average "one mid- to upper-level IS leader every two days" since May.
However, the death of Emwazi would be a "significant blow" to IS, even though he had not been an "operational or major figure" for IS.
Speaking outside Downing Street, Mr Cameron said it was not certain whether the strike was successful, but the UK had been working with the US "literally around the clock" to track Emwazi down.
He said any successful attack would have been "a strike at the heart" of Islamic State (IS), and would demonstrate to those who would harm Britain that "we have a long reach, we have unwavering determination and we never forget about our citizens".
Emwazi appeared in beheading videos of victims including British aid worker David Haines and taxi driver Alan Henning.
Mr Haines's daughter Bethany said she felt an "instant sense of relief" upon hearing reports of Emwazi's possible death, "knowing he wouldn't appear in anymore horrific videos"
"He was only a pawn in Isis's stupid game, but knowing it's over, that he's finally dead, still hasn't sunk in," she said in a statement
"As much as I wanted him dead I also wanted answers as to why he did it, why my dad, how did it make a difference?"
The Henning family were informed on Thursday night by a government contact that the strike had taken place, the BBC understands.
Stuart Henning, Mr Henning's nephew, wrote on Twitter: "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."
Labour leader Jeremy Corbyn said it appeared Emwazi had been "held to account for his callous and brutal crimes".
"However, it would have been far better for us all if he had been held to account in a court of law," he said.
Mohammed Emwazi - timeline
1988: Born in Kuwait, moves to UK in 1994 aged six
Educated at the Quintin Kynaston Community Academy in St John's Wood, north London
Fluent in Arabic and English and a British citizen
2009: Completes computing degree at University of Westminster, travels to Tanzania, Amsterdam and Kuwait
2013: Tries to travel to Kuwait but is stopped. Disappears. Parents report him missing. Police tell family four months later he has entered Syria
Source: Cage, London-based campaign group