The United States said Friday that it has not yet seen any proof to confirm a claim by the Islamic State group that a female American hostage has been killed in an air strike in Syria by a Jordanian plane.
The family of Kayla Mueller, 26, meanwhile urged media to be restrained in their reporting of the IS 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 Carl Mueller.
Mueller was taken captive in August 2013 in the Syrian city of Aleppo, after leaving a Medecins Sans Frontieres (Doctors Without Borders) hospital, according to the family statement, released via Arizona senator John McCain.
MSF noted that she had been visiting the facility with a friend contracted to do some repairs, and was detained as she headed to the bus station in Aleppo, from which she was meant to depart for Turkey.
In a statement posted on jihadist websites, the IS group 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 lived and worked with humanitarian aid groups in northern India, Israel and the Palestinian territories, before returning home to Arizona in 2011 and working at an HIV/AIDS clinic and a women`s shelter.
The suffering of Syrian refugees prompted her to head to the Turkey-Syria border in December 2012 to work with the Danish Refugee Council and the humanitarian group 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 worked as an au pair in France to learn the language, while preparing to work in Africa.
"The common thread of Kayla`s life has been her quiet leadership and strong desire to serve others," said the family statement.
The IS group 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 IS, vowed a harsh response after the jihadists released a gruesome video this week showing the burning alive of airman Maaz al-Kassasbeh.
The IS claim came as Amman said dozens of its jet fighters had struck IS targets, widening their campaign from Syria to include targets in neighbouring Iraq.
Britain-based monitor the Syrian Observatory for Human Rights said more than 30 IS jihadists were killed in coalition raids Friday around Raqa, the "capital" in Syria of the jihadists` self-proclaimed caliphate.