The United States targeted British Islamic State leader "Jihadi John" in an air attack in northern Syria which Britain said would "strike at the heart of Islamic State" if the militant's death is confirmed.

British Prime Minister David Cameron said the death of Mohammed Emwazi, who was known as Jihadi John after appearing in videos showing the killings of U.S. and British hostages, could not yet be confirmed and the Pentagon said it was still assessing the effectiveness of the strike.

But a U.S. official said Thursday's attack in the town of Raqqa, Islamic State's de facto capital, probably killed Emwazi and the Syrian Observatory for Human Rights, a monitoring group, quoted sources in Raqqa as saying he had been blown to pieces.

"A car carrying four foreign Islamic State leaders, including one British Jihadi was hit by U.S. air strikes right after the governorate building in Raqqa city," Rami Abdulrahman, Director of the UK-based Observatory told Reuters.

He initially quoted sources in Raqqi as saying the body of "an important British Jihadi" was in the town's hospital but later quoted them as saying Emwazi's body, and those of his three militant comrades, had been left in charred pieces.

E‎mwazi took part in videos showing the murders of U.S. journalists Steven Sotloff and James Foley, U.S. aid worker Abdul-Rahman Kassig, British aid workers David Haines and Alan Henning, Japanese journalist Kenji Goto, and other hostages.