The United States says it has not yet seen any proof to confirm a claim by Islamic State (IS) militants that an American hostage has been killed in an air strike in Syria.

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 Raqqa, the self-proclaimed capital of the militant group.

"The plane from the crusader coalition bombed a position outside the city of Raqqa after Friday prayers," the statement said.

"The air assaults were continuous on the same location for more than an hour.

"No fighter was wounded but we can confirm that an American hostage was killed in the strikes."

National Security Council spokeswoman Bernadette Meehan said the US was "obviously deeply concerned by these reports" but added that the government had not "seen any evidence that corroborates ISIL's claim".

Who is Kayla Mueller? A 26-year-old aid worker from Prescott, USA

A 26-year-old aid worker from Prescott, USA Was taken hostage while leaving a hospital in the northern Syrian city of Aleppo in August 2013

Was taken hostage while leaving a hospital in the northern Syrian city of Aleppo in August 2013 Has a long record of volunteering abroad and was moved by the plight of civilians in Syria's civil war

Has a long record of volunteering abroad and was moved by the plight of civilians in Syria's civil war Was quoted in her local newspaper The Daily Courier in 2013 as saying: "For as long as I live, I will not let this suffering be normal ... [I will not let this be] something we just accept."

Was quoted in her local newspaper The Daily Courier in 2013 as saying: "For as long as I live, I will not let this suffering be normal ... [I will not let this be] something we just accept." Worked for a Turkish aid organisation on the Syrian border and volunteered for schools and aid organisations abroad including in both the West Bank and Israel, as well as in Dharamsala, India, where she taught English to Tibetan refugees.

Militants named the hostage as 26-year-old aid worker Kayla Mueller, known to have been held by the group since her capture in August 2013.

But the statement provided no evidence to back the claim she had been killed. There were no pictures of a body.

Ms Mueller's parents, Carl and Marsha Mueller, released a statement that said they were still hopeful she was alive and asked the IS group to contact the family privately.

"You told us that you treated Kayla as your guest, as your guest her safety and well-being remains your responsibility," they said in a message directed to "those in positions of responsibility for holding Kayla".

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.

Mr and Mrs Mueller said their daughter devoted her career "to helping those in need in countries around the world" since graduating in 2009.

The suffering of Syrian refugees prompted Kayla to travel to the Turkey-Syria border in December 2012 to work with the Danish Refugee Council and the humanitarian group Support to Life, they said.

US State Department spokeswoman Marie Harf told reporters the department acknowledged that some Americans were held abroad, including by IS, but added: "I cannot confirm those reports in any way."

After a video was released showing the captured Jordanian pilot, Mouath al-Kasaesbeh, being burned alive in a cage Jordan immediately vowed to intensify military action against Islamic State.

IS jihadists have seized swathes of territory in Iraq and Syria, ruling with a brutal version of Islamic law.

The 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.

ABC/wires