(MENAFN- Arab News) WASHINGTON: The parents of an American hostage who the Islamic State group claimed had been killed in a US-led coalition air strike have said they are 'hopeful' she is still alive.

The IS militants 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 US 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 militants' 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 IS group said.

'No fighter was wounded but we can confirm that an American hostage was killed' it said on IS 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 neighboring Iraq.

Jordan is part of the international coalition battling the 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.

Thousands of Jordanians marched Friday to demand retribution against IS for the horrifying murder of pilot Maaz Al-Kasaasbeh.

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 organization' he said.

Amman's government spokesman Mohammad Al-Momani dismissed the IS 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 Kasaasbeh'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 Kasaasbeh who was taken prisoner in December after his F-16 crashed in Syria.