It was the second reported killing of a senior ISIS figure in the last eight days after the group's second-in-command was killed in a US air strike in Iraq on August18.
The source indicated that the US Defence Department was likely involved in the drone strike that killed British hacker Junaid Hussain, a former Birmingham, England, resident.
A CSO Online report said the strike took place on Tuesday near Raqqa, Syria. Hussain, aged 21, moved to Syria sometime in the last two years.
US and European government sources told Reuters earlier this year that they believed Hussain was the leader of CyberCaliphate, a hacking group which in January attacked a Twitter account belonging to the Pentagon, though the sources said they did not know if he was personally involved.
While the American sources said they were confident he was killed in the strike, some people disputed that view. Two Twitter accounts that US intelligence experts say are connected to ISIS reported that his wife had said he was still alive.
Seamus Hughes, a former US government counterterrorism expert now affiliated with George Washington University, said that while the reports came from Twitter accounts known to be connected to ISIS, it was not possible to determine whether they were accurate.
In 2012 he was jailed for six months for stealing former British Prime Minister Tony Blair's address book from an account maintained by a Blair adviser.
Hussain recently had become a subject of considerable US interest. However, the sources denied a British news report that he was No 3 on a drone target list, saying other ISIS commanders were regarded as far more dangerous.