The Islamic State of Iraq and the Levant group (ISIL) group has published a video that purportedly shows the burning death of the captive Jordanian pilot Moaz al-Kassasbeh.
The video released on Tuesday said that the killing of the 26-year-old air force first-lieutenant was in response to Jordan's role in the US-led coalition against ISIL.
Jordan's King Abdullah, who was in Washington DC to meet top US officials, cut short his visit following the news of the murder.
"Today we stand together with the family of Moaz al-Kassasbeh," he said in a video statement.
"This will only make us more united and powerful."
A Jordanian army spokesperson pledged to avenge Kassasbeh's death, saying the response "will be proportional to this catastrophe that has struck all Jordanians.
"His blood will not be shed in vain."
The country's military has officially informed Kassasbeh's family of his death, Al Jazeera's Nisreen el-Shamayleh reported from Amman.
'Very clear message'
According to state television, Jordan has declared a period of mourning. It also reported that Kassasbeh, who had been in ISIL's captivity since December, was killed on January 3.
Speaking to Al Jazeera, Abdel Bari Atwan, a Middle East analyst, said that if proven true, the killing of Kassasbeh was "unprecedented".
"It's a very clear message and they are trying to show maximum brutality," he said, adding that ISIL was not interested in negotiations.
"They are looking to terrorise."
Reacting on the report, US President Barack Obama said that his government and coalition partners will redouble efforts "to make that that they [ISIL] are degraded and defeated".
He said the killing of Moaz "is just one more indication of the viciousness and barbarity of this organisation".
In Kassasbeh hometown, al-Karak, friends, family and supporters gathered to show their support and express anger at the news of his death.
The ISIL video, which is 22 minutes and 34 seconds, also called for the killing of other Jordanian pilots.
Planned prisoner swap
Kassasbeh was taken hostage in ISIL's stronghold of Raqqa in Syria after his F-16 jet crashed.
ISIL members have claimed to have shot down Kassasbeh's plane with a heat-seeking missile.
ISIL, which controls large chunks of Syria and Iraq, had been demanding the release of Sajida al-Rishawi - a woman held by Jordanian authorities since 2005 - in exchange for Kassasbeh's life.
Rishawi was sentenced to death "for conspiracy to carry out terror acts" after a triple bomb attack on the Radisson SAS hotel in Amman.
Security sources told our correspondent that Rishawi would be executed early on Wednesday in response to the killing of Kassasbeh.
Five other individuals on death row could also be executed.
The convicts have already been reportedly moved to al-Suwaqa Jail, where executions in Jordan are typically carried out, a family member of one of the convicts told Al Jazeera's el-Shamayleh.
Source: Al Jazeera