The White House says it cannot confirm if a female American hostage was killed in a Jordanian air strike in Syria.
In a statement posted on sympathetic websites on Friday, the Islamic State of Iraq and the Levant (ISIL) group said Kayla Mueller was buried beneath rubble after an air raid by a Jordanian fighter jet in Raqqa, in northern Syria.
The statement did not show any pictures of a body and there was no independent confirmation of her reported death.
The US said on Friday that it had not yet seen any proof to confirm ISIL's claim.
Bernadette Meehan, National Security Council spokesperson, said: "We are obviously deeply concerned by these reports. We have not at this time seen any evidence that corroborates ISIL's claim."
Jordan, one of several Arab countries in the US-led coalition battling ISIL, has dismissed the statement as "criminal propaganda".
Jordan promised a harsh response after ISIL released a video this week showing the burning alive of Moaz al-Kassasbeh, a Jordan air force pilot.
Kassasbeh, whose F-16 jet came down in December while conducting air strikes, was believed to have been killed in early January.
Mohammed al-Momani, a Jordanian spokesperson, said on Friday the government is investigating.
"But as a first reaction, we think it's illogical and we are highly sceptical about it," he said.
"How could they identify a Jordanian warplane ... in the sky? What was the American lady doing in a weapons warehouse?"
"It's part of their criminal propaganda. They have lied that our pilot is alive and tried to negotiate, claiming he is alive while they had killed him weeks before."
Mueller's family has urged media to be restrained in their reporting about her purported death.
Mueller, 26, 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.
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 Mueller to head to the Turkey-Syria border in December 2012 to work with the Danish Refugee Council and the humanitarian group Support to Life, the family statement said.
When asked what drove her, Mueller 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."
Veracity in question
ISIL 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.
The claim about Mueller's death came on Friday as Jordan said it had widened its air campaign from Syria to include targets in neighbouring Iraq.
The ISIL statement said Mueller was killed during Muslim prayers - which usually take place around midday - in air strikes that targeted "the same location for more than an hour".
It published photos purportedly of the bombed site, showing a severely damaged three-story building, but offered no proof or images of Mueller.
The statement said no ISIL fighters were killed in the air strikes, raising further questions about the veracity of the claim.
The Jordanian military said its fighter jets carried out a series of attacks on Friday and "destroyed the targets and returned safely".
It did not elaborate.
Activists who monitor the Syrian conflict from inside the country said coalition planes hit several targets on the edges and outskirts of Raqqa in quick succession.
A Raqqa-based collective of anti-ISIL activists known as "Raqqa is Being Slaughtered Silently" said the jets targeted multiple ISIL positions and headquarters in the western and eastern countryside of Raqqa, sending up columns of smoke. Explosions could be heard in the city.
The collective said there were no recorded civilian casualties, and did not mention any ISIL casualties.
The Britain-based Syrian Observatory for Human Rights said dozens of ISIL members were killed in coalition air strikes that targeted a tank and vehicle depot in the area of al-Madajen and at least six other ISIL positions, including a training camp and a prison.