BEIRUT (AFP) - The Islamic State in Iraq and Syria (ISIS) group released a video on Tuesday purportedly showing a captive Jordanian fighter pilot being burned alive, in the militants' most brutal execution yet of a foreign hostage.

The highly produced 22-minute video released online showed images of a man purported to be First Lieutenant Maaz al-Kassasbeh, who was captured in December, engulfed in flames inside a metal cage.

Amman confirmed the death of the 26-year-old, while the video's authenticity has still not been verified, and vowed an "earth-shattering" response.

King Abdullah II cut short a visit to Washington to fly home, state television said.

The military warned that "the blood of the martyr will not have been in shed in vain and... vengeance will be proportional to this catastrophe that has struck all Jordanians".

State television said Kassasbeh had been killed Jan 3, before ISIS offered to spare his life and free a Japanese journalist in return for the release of a female would-be suicide bomber on death row in Jordan.

A security official said the woman, Iraqi Sajida al-Rishawi, and other militants will be executed at dawn on Wednesday.

US President Barack Obama immediately denounced the killing.

"Should in fact this video be authentic, it's just one more indication of the viciousness (and) barbarity of this organisation," Obama said of ISIS.

The United States will "redouble the vigilance and determination on the part of the global coalition to make sure" the group is "ultimately defeated", he added.

The White House said US intelligence was working to confirm the video's authenticity.

The chief of the US-led war on ISIS, General Lloyd Austin, condemned the pilot's murder as "savage" and vowed to "fight this barbaric enemy until it is defeated".

British Prime Minister David Cameron said the "sickening murder will only strengthen our resolve to defeat ISIL", another acronym for the group.

Kassasbeh was captured on Dec 24 after his F-16 jet crashed while on a mission over northern Syria as part of the US-led coalition campaign against the militants.

The video released on Tuesday shows footage of him at a table recounting coalition operations against ISIS, with flags from the various Western and Arab countries in the alliance projected in the background.

It then shows Kassasbeh dressed in an orange jumpsuit and surrounded by armed and masked ISIS fighters in camouflage.

It cuts to him standing inside the cage and apparently soaked in petrol before a masked militant uses a torch to light a trail of flame that runs to the cage and burns him alive.

The release of the video of the pilot's purported murder came days after IS beheaded a second Japanese hostage within a week.

Shiraz Maher, from the International Centre for the Study of Radicalisation at King's College London, described the footage as "simply the most horrific, disgusting thing I have seen from Islamic State in the last two years".

"They clearly want to make a real point. This is the first individual whom they have captured who has been directly involved with the Western coalition in fighting ISIS. It is different from the aid workers... This is an act of belligerence".

"Every time you think they cannot commit anything worse - they open up another trapdoor."

ISIS had vowed to kill the second Japanese, Kenji Goto, and Kassasbeh by sunset on Jan 29 unless Amman handed over Iraqi extremist Rishawi.

Kassasbeh's plane was the first loss of an aircraft since the coalition launched strikes against ISIS last year.

Along with Jordan, Saudi Arabia, the United Arab Emirates and Bahrain are taking part in the coalition air strikes in Syria. Australia, Belgium, Britain, Canada, Denmark, France and the Netherlands are participating in Iraq.

The extremist group seized swathes of territory in Iraq and Syria last year, declaring an Islamic "caliphate" and committing a wave of atrocities.

ISIS claimed in a video released Saturday that it had killed 47-year-old Goto, after previously murdering another Japanese hostage, Haruna Yukawa.

The group had initially demanded a US$200 million (S$270 million) ransom for the Japanese hostages - the same amount Tokyo had promised in non-military aid to countries affected by ISIS.

It had previously beheaded two US journalists, an American aid worker and two British aid workers in similar highly choreographed videos.

Jordan had vowed to do everything it could to save the pilot but had demanded proof he was still alive before handing over Rishawi.

ISIS had previously published what it said was an interview with the pilot in which he said his plane was hit by a heat-seeking missile.

ISIS claimed to have shot down his plane but both Jordan and the United States said it had crashed.

Kassasbeh's family had urged ISIS to release the recently married pilot, with his father Safi asking the militant group to show "mercy".

After the killing of Goto, the UN Security Council condemned the "heinous and cowardly" murder, calling for "the immediate, safe and unconditional release of all those who are kept hostage" by the group.