Pentagon press secretary Peter Cook said overnight on Thursday that the attack was carried out in the "Islamic State" (IS) stronghold of Raqqa in northern Syria.
"We are assessing the results of tonight's operation and will provide additional information as and where appropriate," Cook said. It was not immediately clear, however, whether Mohammed Emwazi was killed in the airstrike.
US and British reports later cited unnamed US officials as saying that the airstrike was "flawless" and that Emwazi was essentially "evaporated."
"We are assessing the results for a final confirmation, but initial indications are that 'Jihadi John' is no longer on this planet," The Wall Street Journal cited a US official as saying.
The British-based Syrian Observatory for Human Rightsreported that four people were killed in a strike in Raqqa late on Thursday. Their identities were not immediatelyconfirmed.
British Prime Minister David Cameron said the airstrike targeting Emwazi was the "right thing to do" as he was "intent on murdering many more people."
"This was an act of self defense," Cameron added.
US Secretary of State John Kerry also confirmed on Friday that US officials were still assessing the strike, but said the operation showed that the "terrorist group's days were numbered."
'The Beatles' terror cell
Emwazi was dubbed by the media as "Jihadi John" after a group of hostages said he was part of a terrorist cell who called themselves "The Beatles." All four men also spoke with British accents.
Believed to be in his mid-20s, Emwazi first appeared in a video showing the beheading of US journalist James Foley in August 2014, and is thought to have since been involved in several other killings.
Two weeks after the Foley video emerged, US hostage Steven Sotloff was killed in the same way, again on camera and by an executioner with a British accent.
Emwazi, a London computer programmer, was born to a stateless family of Iraqi origin in Kuwait. His parents moved to Britain in 1993, after their attempts to obtain Kuwaiti citizenship failed.
ksb/rc (Reuters, AFP, dpa)