Islamic State militants released a video on Tuesday appearing to show a captured Jordanian pilot being burnt alive in a cage, a killing that shocked the world and prompted Jordan to promise an "earth-shaking" response.

A Jordanian official said the authorities would swiftly execute several militants in retaliation, including an Iraqi woman whom Amman had sought to swap for the pilot taken captive after his plane crashed in Syria in December.

Reuters could not immediately confirm the content of the video, which showed a man resembling airman Mouath al-Kasaesbeh standing in a small black cage before being set ablaze.

The furious reaction of the Jordanian authorities made clear they treated it as genuine.

Jordan, which has been mounting air raids in Syria as part of the US-led alliance against Islamic State insurgents, would deliver a "strong, earth-shaking and decisive" response, a government spokesman said.

"The revenge will be as big as the calamity that has hit Jordan," army spokesman Colonel Mamdouh al Ameri said in a televised statement confirming the death of the pilot, who was seized by Islamic State in December.

The fate of Kasaesbeh, a member of a large tribe that forms the backbone of support for the country's Hashemite monarchy, has gripped Jordan for weeks and some Jordanians have criticised King Abdullah for embroiling them in the US-led war that they say will provoke a backlash by militants.

The king cut short a visit to the United States to return home following word of Kasaesbeh's death. In a televised statement, he said the pilot's killing was an act of "cowardly terror" by a deviant group that had no relation to Islam.

Jordan had sought to swap the pilot for Sajida al-Rishawi, the Iraqi woman militant who was sentenced to death for her role in a 2005 suicide bombing in Amman that killed 60 people.

Islamic State had demanded her release in exchange for the life of Japanese hostage Kenji Goto. However, Goto, a veteran war reporter, was later beheaded by the group, with images of his death released in a video on Saturday.

The Jordanian security source said Rishawi would be executed "within hours".

The Jordanian military might also escalate attacks on Islamic State, said retired air force General Mamoun Abu Nowar. "We might even see in a couple of days the rate of sorties increased dramatically. We might have some special operations against their leadership too," he said.

In the Islamic State video, Kasaesbeh is interviewed, describing the mission he was due to carry out before his jet crashed. The video also showed footage of the aftermath of air strikes, with people trying to remove civilians from debris.

A man resembling Kasaesbeh is shown inside the cage with his clothes dampened, apparently with flammable liquid, and one of the masked fighters holds a torch, setting alight a line of fuel which leads into the cage.

The man is set ablaze and kneels to the ground.

Fighters then pour debris, including broken masonry, over the cage which a bulldozer subsequently flattens, with the body still inside. The video shows a desert setting similar to previous videos of killings.

DEMAND FOR REVENGE

In the pilot's hometown of Karak in southern Jordan, people demanded revenge.

"I want to see Sajida's body burnt and all the other terrorists in Jordanian prisons ... Only then will my thirst for revenge be satisfied," said Abdullah al-Majali, a government employee among dozens of demonstrators in the centre of Karak.

Relatives of the pilot also gathered in Karak and urged calm after anti-government protests broke out in the town. They said it was up to the government to take revenge for them.

Jordanian state television said on Tuesday that Kasaesbeh had been killed a month ago, on Jan. 3, and a source close to the Jordanian government said Amman had been picking up intelligence for weeks that the pilot was killed some time ago.

Given that Jordan's own intelligence indicated the pilot was dead, the government decided it could not possibly release the woman absent convincing proof the pilot was still alive, the source said, and such proof never arrived.

The White House said the intelligence community was studying when the video was recorded and that US President Barack Obama had ordered his team to devote all resources to locate other hostages held by Islamic State.

Obama rearranged his schedule on Tuesday to make sure he saw King Abdullah before the Jordanian leader flew back to Amman, a source familiar with the situation said.

A staunch US ally, Jordan is part of the alliance against the Islamic State group that has seized large areas of Syria and Iraq. King Abdullah has defended the campaign against domestic criticism, saying that moderate Muslims need to combat a group whose ideology and brutality have insulted Islam.

The United States announced on Tuesday that it will increase annual aid to Jordan to $1 billion from $660 million to help it pay for the cost of housing refugees from Iraq and Syria and to fight Islamic State.

Obama praised Kasaesbeh for his bravery and said he was "in the vanguard of the effort to degrade and defeat the threat" posed by militants from Islamic State, also known as ISIL.

"Today, the coalition fights for everyone who has suffered from ISIL's inhumanity," Obama said in a statement, adding that the group's "hateful ideology" would be banished to "the recesses of history."

Leaders around the world voiced outrage at the brutal killing. "It is incomprehensible how human beings are capable of such a cruel act," said German Chancellor Angela Merkel.

In the video, the burned man wore orange clothes similar to those worn by other foreign Islamic State captives who have been killed since the US-led coalition started bombing the militants in July.

Islamic State has previously released videos showing the beheadings of five US and British hostages and said that it killed two Japanese captives in the same way. It has meted out the same treatment to many more Arab prisoners, including Syrian government soldiers.

The militants have stepped up the gruesome killings while coming under increased military pressure from the air strikes and a ground campaign by Kurdish and Iraqi troops to reverse their territorial gains in Iraq and Syria.

Islamic State emerged out of al Qaeda in Iraq. It expanded into Syria as the country was plunged into a civil war estimated to have killed 200,000 people since 2011.

The Syrian Observatory for Human Rights monitoring group said on Tuesday that 51 civilians, including children, had been killed by Syrian air force strikes inside the country within the past day.

The US-led coalition says it does not coordinate with the Syrian government of President Bashar al-Assad, described by Washington as part of the problem. It does work with the Iraqi government, which is also fighting Islamic State. - Reuters