A statement purportedly by the Islamic State in Iraq and the Levant (ISIL) said that an American hostage was killed in a Jordanian airstrike on Friday on the outskirts of the northern Syrian city of Raqqa, the group's stronghold.
The statement identified the hostage as Kayla Jean Mueller and said she was killed during Muslim prayers, which usually take place around midday on Fridays, in airstrikes that targeted "the same location for more than an hour."
No ISIL fighters were killed in the airstrikes, the statement read.
It published photos allegedly of the bombed site, showing a severely damaged three-story brown building but no images of Mueller.
The statement could not be independently verified. It appeared on a website commonly used by ISIL and was distributed by ISIL-affiliated Twitter users.
Mueller's family appealed Friday for her safety.
"You told us that you treated Kayla as your guest, as your guest her safety and well-being remains your responsibility," Mueller's family said in a short statement.
U.S. officials said they could not confirm that Mueller was killed. Bernadette Meehan, a spokeswoman for the White House National Security Council, said the U.S. was "deeply concerned" over the report but had not seen "any evidence that corroborates ISIL's claim."
Jordan, which is part of a U.S.-led coalition bombing ISIL targets in Syria and stepped up its attacks after ISIL announced it had killed a captive Jordanian pilot, expressed doubt about the group's account of Mueller's death.
"An old and sick trick used by terrorists and despots for decades: claiming that hostages human shields held captive are killed by air raids," Jordanian Foreign Minister Nasser Judeh tweeted.
Mueller is an aid worker whose identity had not been disclosed out of concern for her safety. A family representative told The Associated Press last year that the 26-year-old was working with humanitarian groups in Syria when she was captured in 2013.
If her death is confirmed, she would be the fourth American to die while being held by ISIL. Three other Americans — journalists James Foley and Steven Sotloff and aid worker Peter Kassig — were beheaded by the group.
Al Jazeera and wire services