By Catherine Philp and Sara Elizabeth Williams

Islamic State militants claimed that a female US hostage was killed by Jordanian airstrikes pounding the terror group in Syria today.

The group said Kayla Jean Mueller, an aid worker, was buried alive when air strikes destroyed a building in the city of Raqqa where she was being held hostage.

“It was confirmed to us the killing of an American female hostage by fire of the shells dropped on the site, and she is Kayla Jean Mueller,” Isis said in a statement translated by the monitoring service Site.

“Allah made their pursuit disappointed and deterred their cunning, and no mujahid was injured in the bombardment, and all praise is due to Allah.”

Jordanian warplanes pounded the militant group in Syria and Iraq for a second day in what the king has vowed will be a “relentless war”after the murder of a young Jordanian pilot.

Activists in Isis’s Syrian stronghold of Raqqa reported multiple strikes today in and around the city, where Flight Lieutenant Moaz al-Kasasbeh was burned to death in a cage by Isis militants.

The Isis post claiming Ms Mueller’s death was accompanied by photos showing a collapsed building where she was said to have died. No official confirmation of her death was immediately available.

Ms Mueller, 26, has not previously been identified as a hostage held by the terror group.

The Associated Press said her family had last year confirmed that the aid worker was captured while working with humanitarian groups in Syria in 2013. It said she had not been previously been named as a hostage due to concerns for her safety.

The aid worker, from Prescott, Arizona, had been working with Syrian orphans in Turkey with the international humanitarian aid agency Support to Life last year.

She told her local newspaper last May that she was driven to work with refugees after hearing about the conflict in Syria and found that she “can’t do enough.”

“For as long as I live, I will not let this suffering be normal. (I will not let this be) something we just accept,” she told the Prescott Daily Courier newspaper.

“It’s important to stop and realise what we have, why we have it and how privileged we are. And from that place, start caring and get a lot done.”

Thousands of protesters have marched in support of Jordan’s military retaliation, chanting “Death to Daesh,” an Arabic acronym for Isis which is despised by the jihadist group.

Queen Rania joined the march in the capital of Amman, said to be the largest public protest the country has ever seen and a testament to the rising sentiment in support of Jordan’s intensified battle against Isis.

Many shouted slogans of loyalty towards King Abdullah, who has vowed “relentless war” against the jihadists following the release of a gruesome video showing Flight Lieutenant Kasaesbeh being burned to death.
The video sparked widespread outrage across the Arab world, including from leading Islamic scholars.

A prominent al-Qaeda preacher, known as the world’s foremost jihadist scholar, went on Jordanian television yesterday to denounce the group as liars and deviants, branding the pilot’s killing as a crime against the Islamic faith.

Abu Mohammed al-Maqdisi’s televised appearance came hours after he was released from three months in detention in Jordan for his previous contacts with Isis.

Media said he was freed as a reward for his efforts to persuade Isis to allow Flight Lieutenant Kasaesbeh to go free but his swift, high-profile denunciation of the jihadist group suggested that was as much a motivation for his release.

In an interview with the Jordanian television station, Roya, al-Maqdisi called the immolation of the captive pilot “not acceptable by any religion and by anyone.”

He harshly criticised a fatwa issued by Isis to justify the pilot’s killing, saying that death by fire was a punishment that could only be imposed by God, a reading reflected by most Sunni scholars.

Al-Maqdisi also spoke of his effort to intervene in the hostage crisis, including contacts he made with Isis’ chief scholar, his former pupil, Turki al-Binali, to try and persuade him to release the pilot in a prisoner exchange.

He expressed anger that Isis leaders “lied and were evasive” leading him to believe that Flight Lieutenant Kasaesbeh was alive when in fact the pilot was already long since dead. “They acted like they were interested but in fact they were not,” he said of the failed negotiations for a prisoner swap. “It never fits the one to be a liar and a jihadi.”

Al-Maqdisi’s intervention is significant because of his stature in the global jihadist movement and his specific links to Isis and its predecessor, al Qaeda in Iraq.

He was a mentor to the earlier group’s founder, Abu Musab al-Zarqawi, before breaking with him over his obsessive focus on the brutal slaying of Shia Muslims. Al-Maqdisi also taught al-Binali, 30, Isis’ top clerical authority, before breaking with him too.

While it succeeded in conquering swathe of Syria and Iraq in mere months, Isis has struggled to achieve theological authority even with the global jihadist movement.

Al-Maqdisi addressed that failing with his assertion that an Islamic caliphate cannot be established simply by announcing it but only through divine authority.

His intervention was met with dismay by many Isis supporters, members of a younger generation of jihadists, who denounced him as a traitor on social media, or insisted that he must be speaking under duress.