The US State Department says it cannot confirm reports that a female American hostage has been killed in a recent air strike in Syria targeting Islamic State.
"I cannot confirm those reports in any way - obviously people are looking into them," spokeswoman Marie Harf told reporters.
She acknowledged that Islamic State militants were holding American hostages.
"There are a number of Americans being held overseas, including by Islamic State."
Following confirmation of the US hostage's death, Kayla Mueller's family have released a copy of a letter she wrote in captivity in 2014.
Here is what we know about American aid worker Kayla Mueller who was being held by Islamic State.