The father of the Jordanian pilot who was burned alive by Islamic State has told called his son's killers are "wild beasts".

His condemnation came as Jordan's King Abdullah pledged a "relentless war" against the extremists as he flew home early from Washington for crisis talks over the response to the murder of Mu'ath Al Kassasbeh.

His country's response was swift, taking revenge by executing two prisoners after a horrific video of Mr Al Kassasbeh's killing was published online.

One of them was the Iraqi would-be suicide bomber Sajida al Rishawi, who the extremists had wanted freed.

Safi, the dead pilot's father, told Sky News: "My feeling is that of every father - I am mourning my own son.

"The whole world, the Arab world, the Jordanian society is mourning with me.

"What they did is more than criminal - it's never been seen before in history.

"Even animals couldn't do this - they are wild beasts, they have no connection to Islam.

"They are not human beings let alone Muslims.

"I think the reaction of Jordan will be very strong."

There has been widespread condemnation of the fighter pilot's death, with Saudi Arabia's new King Salman describing his murder as "inhuman and contrary to Islam".

Mr Al Kassasbeh was captured by the militants in December when his F-16 crashed near Raqqa, Syria, the de facto capital of the Islamic State group's self-styled caliphate.

The 26-year-old's murder appeared to be aimed at pressuring the government of Jordan - a close US ally - to leave the coalition that has carried out months of airstrikes on IS positions in Syria and Iraq.

But experts say the brutality shown by the extremists against a fellow Muslim could backfire and serve to galvanise opposition against the militants.