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