The Jordanian air force pilot filmed being burned to death by Islamic State extremists was heavily sedated and unaware of what was about to happen, it has been claimed.

A report in Saudi Arabia's burnews.com claims ISIS militants admitted giving Moaz al-Kasasbeh drugs so that he did not scream as flames consumed his body during his grotesque execution.

Jordan has struck back hard following the circulation of the video last week, hitting the militant group's territory with 56 air strikes in three days and executing two ISIS-linked prisoners.

Burnews.com reports that Kasasbeh's apparent resignation to his fate, as well as his apparently heroic stoicism as his petrol-soaked jumpsuit burst into flames, was due to heavy sedation.

The report also cites unspecified 'observers' it says conducted a detailed study of the video, who concluded that that Kasasbeh's 'sensory centres' burned quickly, leaving him unable to feel the pain of the inferno.

'It is important to note that [Kasasbeh] seemed unconscious and unaware of what awaits him and not, as some have said, that he is not afraid,' the site quotes these observers as saying.

Titled 'Healing the Believers' Chests', the 22-minute film posted last week showed the captured airman locked in a cage before a trail of petrol leading up to its bars is set alight.

Officials believe Kasasbeh had been killed almost one month earlier, despite ISIS attempting to carry out a prisoner exchange in return for the captured pilot.

After the footage was released, Jordanian officials promised to retaliate harshly and quickly executed two Iraqi militants connected with ISIS.

This included Sajida al-Rishawi, the female would-be suicide bomber whose freedom ISIS had originally demanded in exchange for releasing Kasasbeh.

King Abdullah II later said Jordan'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'.

The United States and several Arab allies, including Jordan, have been striking ISIS in Syria since September 23, after the militant group seized control of large areas of Iraq and Syria last year and declared a 'caliphate'.

Warplanes from the U.S. and other countries have been waging an air campaign against the extremists in Iraq for even longer.