The IS jihadists said Kayla Jean Mueller had been buried under rubble when a Jordanian warplane hit a building in the Syrian city of Raqa, their self-proclaimed “capital”.
But the United States said there was no proof that the 26-year-old aid worker from Arizona had been killed.
Her parents appealed to her captors to contact them and for her safe return, in a statement carried by NBC News.
“This news leaves us concerned, yet, we are still hopeful that Kayla is alive. We have sent you a private message and ask that you respond to us privately,” said Carl and Marsha Mueller.
They said they had previously been in touch with IS following her kidnapping in August 2013.
“You told us that you treated Kayla as your guest, as your guest her safety and wellbeing remains your responsibility,” they said.
Jordan — still reeling from the brutal murder of one of its pilots by IS — rejected the jihadists’ claim that its warplanes killed Mueller, calling it an “old and sick trick” to deter coalition strikes.
“The plane from the crusader coalition bombed a position outside the city of Raqa after Friday prayers,” the Islamic State group said.
“No fighter was wounded but we can confirm that an American hostage was killed,” it said on jihadist websites without publishing any pictures of her body.
The claim came as Amman said its fighter jets had launched dozens of strikes on IS, widening their campaign from Syria to include targets in neighbouring Iraq.
– ‘Turning point’ –
Jordan is part of the international coalition battling the Sunni extremist group, which has seized swathes of territory in Syria and Iraq and imposed an extreme interpretation of Islam on the areas under its control.
Interior Minister Hussein Majali said in remarks published Saturday that the burning alive of a Jordanian pilot by IS marked a “turning point” in the kingdom’s fight against extremism.
US authorities have never given figures on the number of Americans kidnapped in Syria, sticking to a policy of complete silence.
The US National Security Council said it has not “seen any evidence that corroborates” the IS claim that Mueller had been killed.
Mueller’s family said she had “devoted her career to helping those in need in countries around the world” including the Syrian people.
She travelled to the Syrian-Turkish border in 2012 to help refugees fleeing the civil war and was captured in Aleppo after leaving a Doctors Without Borders hospital.
– ‘Old and sick trick’ –
Thousands of Jordanians marched Friday to demand retribution against IS for the horrifying murder of pilot Maaz al-Kassasbeh.
Queen Rania joined them after weekly prayers at the Al-Husseini mosque, holding a portrait of the pilot with the words “Maaz the martyr of righteousness”.
She told the BBC that the battle against IS “is absolutely Jordan’s war,” but that “to win it we need help from the international community.”
The kingdom’s airstrikes are “the beginning of an ongoing process to eliminate” IS, Interior Minister Majali was quoted as saying by government newspaper Al-Rai.
“The day of the hero, martyr pilot’s assassination is a turning point in Jordan’s history in order to face this horrific crime that was committed by the cowardly terrorist organisation,” he said.
Amman’s government spokesman Mohammad al-Momani dismissed the jihadists’ claim as “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,” Momani told AFP.
The Syrian Observatory for Human Rights, a British-based monitor, said more than 30 IS fighters were killed in coalition raids Friday around Raqa.
IS had offered to spare Kassasbeh’s life and to exchange Japanese hostage Kenji Goto for Iraqi militant Sajida al-Rishawi, who was on death row for her role in deadly hotel bombings in Amman in 2005.
Goto was later beheaded and Jordan hanged Rishawi after IS released the video showing the murder of Kassasbeh, who was taken prisoner in December after his F-16 crashed in Syria.
Jihadists have flocked to Syria during a nearly four-year-old war in which more than 210,000 people have died, according to an updated Observatory toll.- AFP