Isis US hostage death: Militants claim female American aid worker Kayla Jean Mueller has been killed by Jordanian air strikes

American officials are desperately trying to confirm the veracity of a claim by Isis which said a female US hostage being held by the militants had been killed by a Jordanian air strike.

In a message posted on social media, Isis claimed that Kayla Jean Mueller died when Jordanian planes struck targets close to Raqqa, the de facto capital of Isis-controlled territory. Ms Mueller, an aid worker from Arizona, was taken hostage in Syria in August 2013.

The message from the militants was subsequently published by the SITE Intelligence Group, which track extremists activity.

“The failed Jordanian aircraft killed an American female hostage,” said the message. “No mujahid was injured in the bombardment, and all praise is due to Allah.” The word mujahid means fighter.

It said the woman had been killed by “fire of the shells dropped on the site”, said the New York Times. Isis also published photographs of the purported bombed site, showing a severely damaged brown coloured three-story building, but none of Ms Mueller.

There was no independent confirmation of the claim. US media on Friday morning that various US agencies were aware of the report and were trying to obtain further information. The Jordanian foreign minister, Nasser Judeh, said on his Twitter: "[This is] an old and sick trick used by terrorists and despots for decades: claiming that hostages human shields held captive are killed by air raids.”

In Washington, Marie Harf, a spokeswoman for the US Department of State, told reporters she could offer no information about the Isis claim but said “obviously people are looking into it”.

She said there were a number of American citizens being held hostage around the world, including by Isis. She said Secretary of State John Kerry and others were doing all they could to locate these people and secure their release. “These are very sensitive situations,” she added.

The fact that Ms Mueller was being held by Isis, had been known for some time, though few details had been publicly released. Her family had requested that her identify not be made public, on the advice of US officials.

It is understood she was seized in August 2013 while working with adults and children in Aleppo who had been displaced by Syria’s ongoing civil war. Last year, Isis demanded $6m for her release. As part of the deal the group also demanded the release of convicted Pakistani scientist Aafia Siddiqui.

Last month, President Barack Obama’s chief of staff, Denis McDonough, erred when he mentioned the woman’s name during a talk show interview. All references to the woman were later deleted.

The Associated Press said that when they interviewed Ms Mueller’s family last year they confirmed she had been doing humanitarian work in Syria when she was captured. Several other people were taken hostage at the same time.

Jordan had stepped up its bombing campaign against Isis in the aftermath of the release of video earlier this week which showed the burning alive of a Jordanian pilot, Lt Muath al-Kaseasbeh, whose plane was downed in December while on an operation against militant targets.

Jordan had been trying to arrange the exchange of Mr Kaseasbeh for several militants it was holding. But when Isis released footage of the killing of the pilot - whom it transpired had been killed in January and who was dead while Isis was claiming to negotiate his release - Jordan went ahead and executed two militants.

One of them was Sajida al-Rishawi, the Iraqi woman militant who was sentenced to death for her role in a 2005 suicide bombing in Amman that killed 60 people. The other was Ziad Karbouli, a former aide to the deceased leader of al-Qaeda in Iraq, Abu Musab al-Zarqawi.

In the aftermath of the killing of Mr Kaseasbeh, King Abdullah of Jordan vowed revenge and ordered his air force to increase their strikes on Isis targets. Jordan said in September it was joining the US-led coalition against Isis, along with Bahrain, Qatar, Saudi Arabia and the United Arab Emirates.

Video footage released by the Jordanian military showed some of the strikes, including one that set off a fire ball. Elsewhere pilots could be seen chalking messages on their missiles, including one that read: “For you, the enemies of Islam.”

A statement read on Jordanian state television declared: “This the beginning and you will get to know the Jordanians.”

If the death of Ms Mueller confirmed, she would be the fourth American to die while in the captivity of Isis militants. Three other Americans - journalists James Foley and Steven Sotloff and aid worker Peter Kassig -were beheaded by the group.

Two British hostages, David Haines and Alan Henning, were also killed by Isis.

Follow Andrew Buncombe on Twitter: @AndrewBuncombe