British Prime Minister David Cameron says officials are not yet certain whether a U.S. drone strike killed Mohammed Emwazi, the masked militant in Islamic State-group beheading videos known as "Jihadi John."

American officials say a U.S. drone strike targeted a vehicle in Syria believed to be transporting Emwazi on Thursday.

Cameron said Friday the strike was a joint effort and that British intelligence agencies were working around the clock to find the British-accented militant, whom he called the militant group's "lead executioner."

He said the U.S. strike had been "an act of self-defense" and the right thing to do.

Cameron said targeting Emwazi was "a strike at the heart" of the Islamic State group.