WASHINGTON (AFP) - The United States said Friday that it has not yet seen any proof to confirm a claim by the Islamic State in Iraq and Syria (ISIS) group that a female American hostage has been killed in an air strike in Syria by a Jordanian plane.
The family of Kayla Jean Mueller, 26, meanwhile urged media to be restrained in their reporting of the ISIS claim about the aid worker.
"The family... request that media cautiously report on her background, work and current situation and limit speculation on her situation, and consider the implications for her security before publishing," said a statement by her parents, Marsha and Carla Mueller.
Mueller was taken captive in August 2013 in the Syrian city of Aleppo, while working for a Spanish Medecins Sans Frontieres (Doctors Without Borders) hospital, according to the family statement, released via Arizona senator John McCain.
In a statement posted on militant websites, ISIS said the woman was buried beneath rubble after an air raid by a Jordanian warplane in Raqa.
The statement did not show any pictures of a body and there was no independent confirmation of her reported death.
National Security Council spokeswoman Bernadette Meehan would only say: "We are obviously deeply concerned by these reports. We have not at this time seen any evidence that corroborates ISIL's claim."
Mueller's parents, who live in Prescott, Arizona, said their daughter has devoted her career "to helping those in need in countries around the world" since graduating from Northern Arizona University in 2009.
She was drawn by the suffering of Syrian refugees to the Turkish/Syrian border in December, 2012 to work with the Danish Refugee Council and the humanitarian organisations Support to Life, their statement said.
When asked what drove her, she once said: "I find God in the suffering eyes reflected in mine, if this is how you are revealed to me, this is how I will forever seek you."
Before heading in Syria, she had studied French in France, while preparing to work in Africa.
After graduating in 2011, she lived and worked with humanitarian aid groups in northern India, Israel and Palestine, before returning home to Arizona and working at an HIV/Aids clinic and a women's shelter.
ISIS has murdered both locals and foreigners, including two US journalists, an American aid worker, two British aid workers, two Japanese hostages and a Jordanian pilot.
Jordan, one of several Arab countries in the US-led coalition battling ISIS, vowed a harsh response after the extremists released a gruesome video this week showing the burning alive of airman Maaz al-Kassasbeh.