Islamic State said on Friday that an American woman hostage it was holding in Syria was killed when Jordanian fighter jets bombed a building where she was being held, but Jordan expressed doubt about the Islamist militant group's account of her death.
In Washington, US officials said they could not confirm that the woman, 26-year-old humanitarian 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.
The group has beheaded three other Americans, two Britons and two Japanese hostages - most of them aid workers or journalists - in recent months. Mueller was taken hostage while leaving a hospital in the northern Syrian city of Aleppo in August 2013.
The group's latest claim, detailed by the SITE monitoring group, came 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 a stepped-up military role in the US-led coalition against the group. Jordan said it had carried out a second straight day of air strikes on Friday on Islamic State positions.
"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 of Mueller's family said the family had no information on Islamic State's statement.
White House National Security Council spokeswoman Bernadette Meehan said the United States is "deeply concerned" over the report but had not seen "any evidence that corroborates" the group's account.
Islamic State, in a message monitored by SITE, said Mueller died when the building in which she was being held outside Raqqa, a stronghold of the group, collapsed in a Jordanian air strike on Friday.