WASHINGTON, Nov 12  - The United States on Thursday carried out an air strike in Syria targeting the Islamic State militant best known as "Jihadi John," who participated in gruesome videos of the killings of American and British hostages, officials said.

The Pentagon said it was still evaluating the effectiveness of the strike in the Syrian city of Raqqa, the de facto capital of Islamic State.

If his death is confirmed, it would be an important milestone in the U.S.-led campaign against the group and would come more than a year after U.S. President Barack Obama promised justice for 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 Islamic State brutality and one of the world's most wanted men.

His real name was Mohammed Emwazi, and he was a British citizen and computer programing graduate, hailing from a well-to-do London family.

Emwazi appeared in videos released by Islamic State last year that showed the killings of U.S. journalists James Foley and Steven Sotloff and U.S. aid worker Peter Kassig, who had been held hostage.

Kassig, from Indiana, was also known as Abdul-Rahman, a name he took after converting to Islam in captivity.

The videos also showed the deaths of British aid workers David Haines and Alan Henning, Japanese journalist Kenji Goto, and other hostages.

Emwazi used the videos to threaten the West, admonish its Arab allies and taunt Obama and British Prime Minister David Cameron in front of petrified hostages cowering in orange jump suits.

"We are assessing the results of tonight's operation and will provide additional information as and where appropriate," said Pentagon spokesman Peter Cook.

U.S. officials, speaking on condition of anonymity, said it was too soon to be certain whether Emwazi was killed in the strike. It was not immediately clear how long it might take to determine that.