In a public statement, the parents of US hostage Kayla Mueller have asked Islamic State (Isis) to contact them directly.

The parents of the 26-year-old aid worker said they had decided to break their public silence at news of reports of their daughter's death during air strike by Jordan, and they are "hopeful" she is still alive.

"This news leaves us concerned, yet, we are still hopeful that Kayla is alive," they said. "We have sent you a private message and ask that you respond to us privately. You told us that you treated Kayla as your guest, as your guest her safety and wellbeing remains your responsibility."

Isis has stated that she was killed by a Jordanian air strike in Syria. White House officials have said they have seen no photographic proof to prove she was killed.

Isis has released images of a ruined building where it claims Mueller died, but has not supplied any photos of the aid worker's body.

It claimed that Jordanian jets bombed the building, which was sited on the edge of Raqqa, the main Syrian city in Isis's possession.

There are problems with Isis's claims. The photos of the damaged building bore Thursday's date, despite their claim that the bombing took place during Friday prayers. Isis also claimed Mueller was the only person who died in the airstrike and that none of its fighters were injured or killed. Jordan did carry out air strikes on IS targets in Syria on Thursday, but it has dismissed the reports about Mueller's alleged death as propaganda.

Jordan did carry out air strikes on Isis targets in Syria on Thursday, and has dismissed the reports about Mueller's alleged death as propaganda.

Mueller, 26, came to the Turkish and Syrian border in 2012 to work with refugees. She was kidnapped the following year while she was working in Aleppo, Syria, after leaving a Spanish-run Doctors Without Borders hospital.

If her death is confirmed, she will be the fourth US citizen to die after being captured by Isis. Journalists James Foley and Steven Sotloff, and aid worker Peter Kassig, have all been beheaded.