A "British" leader of Islamic State and three other foreign militants have been killed in U.S.-led air strikes in northern Syrian town of Raqqa, monitoring group the Syrian Observatory for Human Rights said on Friday.

The Observatory did not confirm the British national was the militant known as "Jihadi John", after a U.S. official said early on Friday that a strike in the area probably killed him.

"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.

"All the sources there are saying that the body of an important British Jihadi is lying in the hospital of Raqqa. All the sources are saying it is of Jihadi John but I cannot confirm it personally."