WASHINGTON — A U.S. drone strike targeted a vehicle in Syria believed to be transporting the masked Islamic State militant known as “Jihadi John,” according to American officials. Whether the strike killed the British man who appears in several videos depicting the beheadings of Western hostages was not known, British Prime Minister David Cameron said today.

Mohammed Emwazi was the target of an airstrike in Raqqa, Pentagon press secretary Peter Cook said in a statement. Officials were assessing the results of the strike, he said.

A U.S. official told The Associated Press that a drone had targeted a vehicle in which Emwazi was believed to be traveling.

Cameron said officials are not yet certain whether a U.S. drone strike killed Emwazi. He said the strike had been a joint effort and that British intelligence agencies were working around the clock to find the British-accented militant, whom Cameron called the militant group”s “lead executioner.”

Cameron also said the U.S. strike had been “an act of self-defense” and the right thing to do. He said targeting Emwazi was “a strike at the heart” of the Islamic State group.

U.S. Secretary of State John Kerry, appearing at a news conference in Tunis, Tunisia today said, “We are still assessing the results of this strike.” He told reporters extremists “need to know this: your days are numbered and you will be defeated.”

Peter Neumann, professor of security studies at King’s College London, said U.S. and British authorities appeared fairly confident Emwazi was dead. He said officials would be wary of announcing he’d been targeted only for him to appear “on video in a day’s time sticking his finger up at Obama and saying ‘I’m still alive.’ “

He said that Emwazi was not a senior military commander of ISIS but was “very important to the organization symbolically.”

The Britain-based Syrian Observatory for Human Rights said that U.S. coalition warplanes struck an IS vehicle as it left the governor’s office in the city of Raqqa, killing four IS foreign fighters, including a British commander. The Observatory said the bodies were charred, and Observatory chief Rami Abdurrahman said the commander killed in the attack was most likely Jihadi John but that he does not have 100 percent confirmation.

Emwazi, believed to be in his mid-20s, has been described by a former hostage as a bloodthirsty psychopath who enjoyed threatening Western hostages. Cameron said Britain has been “working, with the United States, literally around the clock to track him down.”