moaz al-kasabeh
A man purported to be Islamic State captive Jordanian pilot Moaz al-Kasasbeh is seen standing in a cage in this still image from an undated video filmed from an undisclosed location made available on social media on February 3, 2015.Reuters

The Jordanian pilot filmed being burned to death was heavily sedated so he would not scream as flames consumed his body, it has emerged.

The horrific execution of Moaz al-Kasasbeh shocked the world when Islamic State (Isis) militants released a video of his death last week. The 26-year-old was doused in petrol and set alight before being buried under rubble by a bulldozer.

A report on Saudi Arabia's claims IS militants admitted giving al-Kasabeh drugs to make him unaware of what would happen to him.

"It is important to note that [Kasabeh] seemed unconscious and unaware of what awaits him and not, as some have said, that he is not afraid," the site reported.

The video entitled Healing the Believers Chests was posted last week showed the pilot locked in a cage before he was set on fire.

Officials now believe Kasasbeh may have been killed almost a month earlier despite negotiations with IS to carry out a prisoner exchange for Sajida al-Rishawi - the female suicide bomber - in return for the pilot. She has since been executed along with another Iraqi militant connected with IS.

Jordan has also increased its offensive against IS with a series of air strikes.

Jordan's King Abdullah II said his country's response would "be harsh because this terrorist organisation is not only fighting us, but also fighting Islam and its pure values".

In a statement, he pledged to hit the militants "hard in the very centre of their strongholds".

Meanwhile it has been reported members of IS are defecting or deserting the terror regime.

Abu Mohammed told Raqqa Is Being Silently Slaughtered: "There is great tension in Raqqa city, where IS group suffered many defections in the past few days, a source from within IS has confirmed that most of defections are from suicide bombers, where these defections are considered a painful blow to the group."