In Washington, US officials said they could not confirm that the woman, who has been identified as 26-year-old aid worker Kayla Mueller of Prescott, Arizona, had been killed.
Mueller was the last-known American hostage held by Islamic State, which controls wide areas of Syria and Iraq and has executed five British and American aid workers and journalists in recent months.
The group’s latest claim, detailed by the SITE monitoring group, comes just days after it released a video on Tuesday showing a captured Jordanian pilot, Mouath al-Kasaesbeh, being burned alive in a cage.
Jordan’s King Abdullah, who was in Washington discussing how to deal with Islamic State militants when the video was made public, vowed to avenge the pilot’s death and ordered commanders to prepare for a stepped-up military role in the US-led coalition against Islamic State militants. Jordan said it had carried out a second straight day of air strikes on Friday on Islamic State militants.
“We are looking into it but our first reaction is that we think it is illogical and we are highly sceptical about it. ... It’s part of their criminal propaganda,” government spokesman Mohammad Momani said in response to Islamic State’s account of what happened to Mueller.
“How could they identify Jordanian war planes from a huge distance in the sky? What would an American woman be doing in a weapons warehouse?” Momani added.
Hours after the release of the video showing the pilot burning to death, Jordanian authorities executed two al Qaeda militants who had been imprisoned on death row, including a woman who had tried to blow herself up in a suicide bombing and whose release had been demanded by Islamic State.
A representative in the United States of Mueller’s family said the family had no information on Islamic State’s statement that she had been killed.
State Department spokeswoman Marie Harf told reporters during a briefing in Washington, “I cannot confirm those reports in any way.”
Bernadette Meehan, a spokeswoman for the White House National Security Council, said the US is “deeply concerned” over the report but had not seen “any evidence that corroborates ISIL’s claim,” using an acronym for the group.
Islamic State, in a message monitored by SITE, said Mueller died when the building in which she was being held outside Raqqa, a major stronghold of the group, collapsed in a Jordanian air strike yesterday.
“The air assaults were continuous on the same location for more than an hour,” Islamic State said, according to SITE.
Reuters and other news organizations were aware Mueller was being held hostage but did not name her at the request of her family members, who believed the militants would harm her if her case received publicity.