By Mark Hosenball

WASHINGTON (Reuters) - A British hacker who U.S. and European officials said became a top cyber expert for Islamic State in Syria has been killed in a U.S. drone strike, a U.S. source familiar with the matter said on Wednesday.

It was the second reported killing of a senior Islamic State figure in the last eight days after the group's second-in-command was killed in a U.S. air strike in Iraq on Aug. 18.

The source indicated that the U.S. Defense 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, 21, moved to Syria sometime in the last two years.

U.S. 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 U.S. intelligence experts say are connected to Islamic State reported that his wife had said he was still alive.

Seamus Hughes, a former U.S. government counterterrorism expert now affiliated with George Washington University, said that while the reports came from Twitter accounts known to be connected to Islamic State, it was not possible to determine whether they were accurate.

"It could be a concerted attempt to deceive," Hughes said.

Cyber security experts have said they believe that Hussain and other hackers working for Islamic State lack the skills needed to launch serious attacks such as ones that could shut down computer networks or damage critical infrastructure.

"He wasn't a serious threat. He was most likely a nuisance hacker," said Adam Meyers, vice president of cybersecurity firm CrowdStrike. "It was his involvement in recruitment, communications and other ancillary support that would have made him a target."

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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 U.S. interest. However, the sources denied a British news report that he was No. 3 on a drone target list, saying other Islamic State commanders were regarded as far more dangerous.

(Reporting by Mark Hosenball; Additional reporting by Jim Finkle in Boston; Editing by Eric Beech)