WASHINGTON (AP) — A U.S. drone strike targeted a vehicle in Syria believed to be transporting the masked Islamic State militant known as "Jihadi John," U.S. officials said, but it was still unclear whether the strike killed the British man who appears in several videos depicting the beheadings of Western hostages.

Mohammed Emwazi, a Kuwaiti-born British citizen, 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.

U.S. military spokesman Steve Warren said officials were "reasonably certain" they had killed Jihadi John with a Hellfire missile fired from a drone. Another U.S. official told The Associated Press that a drone had targeted a vehicle in which Emwazi was believed to be traveling. The official was not authorized to speak publicly and requested anonymity.

Warren said the world is better off without the man believed to have beheaded several Western hostages, whom he referred to as a human animal. He said the operation was one in a string of targeted attacks on Islamic State leaders. He says the U.S. has killed one mid- to upper-level ISIL leader every two days since May.

U.S. Secretary of State John Kerry, appearing at a news conference in Tunis, Tunisia, on Friday told reporters extremists "need to know this: Your days are numbered, and you will be defeated."

The Britain-based Syrian Observatory for Human Rights said that U.S. coalition warplanes struck an Islamic State vehicle as it left the governor's office in the group's self-proclaimed capital of Raqqa, Syria, 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. Spanish journalist Javier Espinosa, who had been held in Syria for more than six months after his abduction in September 2013, said Emwazi would explain precisely how the militants would carry out a beheading.

Those being held by three British-sounding captors nicknamed them "the Beatles," with "Jihadi John" a reference to Beatles member John Lennon, Espinosa said.

Among those beheaded by Islamic State militants in videos posted online since August 2014 were U.S. journalists Steven Sotloff and James Foley, U.S. aid worker Abdul-Rahman Kassig, British aid workers David Haines and Alan Henning, and Japanese journalist Kenji Goto.

Their friends and relatives all said Friday that even if Emwazi was dead, it would bring little comfort.

A friend of Henning's has said she is still "skeptical" following news that "Jihadi John" may have been killed. Foley's parents, John and Diane Foley, of New Hampshire, issued a statement calling Emwazi's purported death "a very small solace."

"His death does not bring Jim back." ''If only so much effort had been given to finding and rescuing Jim and the other hostages who were subsequently murdered by ISIS, they might be alive today," said the Foleys' statement, using an alternate acronym for the Islamic State.

In the videos, a tall masked figure clad in black and speaking in a British accent typically began one of the gruesome videos with a political rant and a kneeling hostage before him, then ended it holding an oversize knife in his hand with the headless victim lying before him in the sand.