ISIL militants in Syria claim that Jordanian airstrikes have killed a US woman hostage.
The group claims humanitarian worker Kayla Mueller, a 26-year-old from Arizona, was in a building that was hit.
Jordan has voiced doubt about the report, and Washington is investigating.
The US National Security Advisor, Susan Rice, said: “We are obviously very concerned about the reports that have come in in recent hours.
“We do not at present have any evidence to corroborate ISIL’s claims, but obviously we will keep reviewing the information.”
Photos that allegedly show the aftermath of the Jordanian airstrikes have appeared online.
Kayla Mueller is the last known US hostage held by ISIL.
The claims come after militants released a video showing a captured Jordanian pilot being burned alive in a cage.
The murder has angered Jordan, which is now promising a “harsh” and “relentless” war against ISIL.
Dozens of fresh airstrikes on suspected militant positions have been carried out by Jordan since the video was released.