The Islamic State group has claimed that a female American hostage has been killed in an air strike in Syria carried out by the US-led coalition.
In a statement posted on jihadist websites, IS said the woman, who has been identified as Kayla Mueller, was buried beneath the rubble after a raid by a Jordanian warplane in Raqqa, the self-proclaimed capital of the militant group.
The statement did not show any pictures of a body and there was no independent confirmation of her reported death.
It also said that the bombing took place after traditional Friday prayers, adding no fighters were killed in the raid.
However, the Syrian Observatory for Human Rights monitoring group said that at least 30 fighters were killed in and around Raqqa.
The US State Department has said it could not confirm the reports.
Spokeswoman Marie Harf acknowledged that some Americans are being held by IS but did not provide any further information.
A White House spokesperson said they are "deeply concerned" by the reports.
Jordan, one of several Arab countries in the US-led coalition, vowed a harsh response after IS released a gruesome video this week showing the burning alive of Jordanian pilot Maaz al-Kassasbeh.
Kassasbeh was captured by IS in December after crashing in Syria while on a mission.
US very concerned by reports of hostage death
US National Security Advisor Susan Rice has said the country is very concerned about the reports.
Speaking as she launched the Obama Administration's National Security Strategy in Washington DC, Ms Rice said they did not have any evidence to corroborate the claims made by the IS group that the young woman had been killed, but they were monitoring the situation.
She said she stood by the US policy of not offering concessions to terrorists or paying ransom in exchange for hostages.
However, she said a review of the overall hostage policy was currently under way as President Obama felt they could "do better" when it came to handling hostage situations.
She said this review involved examining how other countries dealt with hostage-taking, citing the UK as an example.
Ms Rice said they were also talking to the families of hostages to find out what the administration could have done better for them.
