Jordan says it has carried out a third consecutive day of air strikes against Islamic State in Syria.

It comes after IS militants alleged one of Jordan's raids had killed an American hostage being held by the group - 26-year-old Kayla Mueller.

The US says it has still not seen any real proof she is indeed dead.

Dr Todd Geiler is a friend of the Mueller family:

In a statement released by a family representative, Ms Mueller's parents Marsha and Carl Mueller made a personal appeal to IS.

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

The statement asked IS to contact the family privately.

According to the Site Intelligence Group which monitors extremists, IS said the aid worker (26) died on Friday after Jordanian warplanes struck the building where she was being held.

The terrorist group said no IS fighters died in the raids in their de facto capital of Raqqa, Syria.

It released images showing a damaged building it said had been targeted in airstrikes, but no photos to back their claims Ms Mueller had been killed.

The White House, State Department and Pentagon have said they cannot confirm the unsubstantiated report.

"We are obviously deeply concerned by these reports," said Bernadette Meehan, spokeswoman for the White House National Security Council, in a statement.

"We have not at this time seen any evidence that corroborates ISIL's claim."

Jordan has also dismissed the IS claim as "criminal propaganda", but said its jets did carry out a second consecutive day of strikes on IS sites on Friday.

It has stepped up its operations against IS since the militants burned to death a captured Jordanian pilot.

Ms Mueller, of Prescott, Arizona, disappeared in August 2013 in the northern Syrian city of Aleppo.

A media blackout on her abduction was broken by President Barack Obama on Sunday, when he confirmed IS still had at least one US hostage.

He said the US was "deploying all the assets that we can" to find Ms Mueller. Her name had not been made public due to fears for her safety.

The group has already executed three American hostages: James Foley, Peter Kassig and Steven Sotloff.

Two British hostages - David Haines and Alan Henning - and two Japanese hostages, Kenjo Goto and Haruna Yukawa, have also been killed.

British reporter John Cantlie is still being held captive.