TUNIS (AFP) - United States Secretary of State John Kerry on Friday (Nov 13) warned the Islamic State in Iraq and Syria (ISIS) that its "days are numbered", following an American strike in Syria targeting British militant "Jihadi John".

"The coalition forces conducted an air strike targeting... Jihadi John," whose real name is Mohammed Emwazi, he said on a visit to Tunis.

"We are still assessing the results of this strike but the terrorists associated with Daesh need to know this: Your days are numbered and you will defeated," said Mr Kerry, using the Arabic acronym for ISIS.

"There is no future, no path forward" for ISIS, the secretary of state said.

The Pentagon said the US military conducted an air strike Thursday targeting Emwazi, the masked ISIS militant with a British accent seen in grisly videos executing Western hostages.

"We are assessing the results... and will provide additional information as and where appropriate," said spokesman Peter Cook.

The Pentagon said the air strike took place in Raqqa, the ISIS's de facto Syrian capital.

CNN and the Washington Post, citing officials, reported that Emwazi was targeted by a drone.

The US strike on Syria that targeted British "Jihadi John" was "an act of self defence", Britain's Prime Minister David Cameron said on Friday, while acknowledging his death was "not yet certain".

Mr Cameron said the operation against Mohammed Emwazi, who appears in a string of graphic videos showing the execution of Western hostages, was a combined British-US effort.

"We cannot yet be certain if the strike was successful," Mr Cameron said in a statement delivered outside his Downing Street office.

If it was confirmed, it would be "a strike at the heart of ISIL," he said, using an alternative term for ISIS.

But analysts said the impact of his death would likely be symbolic rather than tactical for the group, which controls swathes of Iraq and Syria and is known for perpetrating widespread atrocities.

"Emwazi, a British citizen, participated in the videos showing the murders of US journalists Steven Sotloff and James Foley, US aid worker Abdul-Rahman Kassig, British aid workers David Haines and Alan Henning, Japanese journalist Kenji Goto, and a number of other hostages," the Pentagon said.

CNN and the Washington Post, citing officials, said Emwazi was targeted by a drone. He was last seen in the video showing Mr Goto's execution in January.

The Syrian Observatory for Human Rights, a Britain-based monitoring group, said four people were killed in a strike in Raqa late on Thursday.

"The car was hit in the centre of town, near the municipality building," Observatory chief Rami Abdel Rahman said, quoting sources who said one of the victims was a "senior British member of the group".

Emwazi, a London computer programmer, was born in Kuwait to a stateless family of Iraqi origin. His parents moved to Britain in 1993 after their hopes of obtaining Kuwaiti citizenship were quashed.

Mr Charlie Winter, an academic who focuses on ISIS activities, said it could be a "big blow".

"Symbolically it's really important. Jihadi John... was someone who was a source of hubris, a sort of an aspirational figure for fighters in Islamic State," he said.

"He was a key figure of defiance in the face of the international coalition, so if in fact he has been killed, this is going to be a big blow."

Court papers published by British media connected Emwazi to a network of extremists known as "The London Boys" that were originally trained by the Shebab, Al-Qaeda's East Africa affiliate.