Media playback is unsupported on your device Media caption The parents of murdered hostage Kassig believe that "good will prevail"

The parents of a US medic beheaded by Islamic State (IS) militants have paid tribute to their son, saying they will learn in time to forgive his captors.

"Our hearts are battered, but they will mend. The world is broken, but it will be healed," said Ed and Paula Kassig, parents of Abdul-Rahman Kassig.

Mr Kassig, 26, is the fifth Western hostage to be killed by IS.

Western intelligence officials are working to identify militants seen in a video showing his beheading.

A French prosecutor said one militant was Maxime Hauchard, 22, and another Frenchman might have been present. A third militant was reported to be a Briton.

"Our beloved son... no longer walks this earth," Ed Kassig told reporters on Monday. "Our hearts, though heavy, are held up by the love and support that has poured into our lives these last few days."

The couple asked for prayers for their son on Monday, as well as for those held against their will in Iraq, Syria and around the world.

Image copyright AP Image caption An undated family photo from Kassig's youth

Image copyright Handout Image caption A photograph of Abdul-Rahman with his father, Ed, fishing on the Ohio River in Indiana in 2011

"Lastly, please allow our family the time and privacy to mourn, cry, and yes, forgive, and begin to heal," Mr Kassig added.

The IS video released on Sunday shows a masked man standing over a severed head, which the White House later confirmed was Kassig's.

He was captured by IS, which controls large parts of Syria and Iraq, in October 2013 while travelling to Deir Ezzour in eastern Syria.

Analysis: Frank Gardner, BBC security correspondent

The 16-minute video posted online by IS, while graphically sadistic, also reveals a wealth of detail about the perpetrators and their whereabouts. This is deliberate. IS is effectively taunting the Western-led coalition that is arranged against it, saying this is who we are, come and get us if you dare.

Intelligence agencies will be using facial recognition software to identify those involved in the mass beheading, matching their real names and origins to their adopted battle names. IS has even put a place name on the video, Dabiq in northern Syria - a place where according to Islamic hadith, an apocalyptic battle will be fought between Muslims and non-Muslims.

The one person who keeps his identity concealed is the suspected British jihadist known in the UK media as "Jihadi John". He is believed to be from London and both the FBI and MI5 almost certainly know who he is but for reasons known to them, they are not revealing it.

An idealist 'simply trying to help people'

Abdul-Rahman Kassig in his own words

US Secretary of State John Kerry said in Washington: "[IS] leaders assume that the world will be too intimidated to oppose them. But let us be clear: We are not intimidated."

UK Prime Minister David Cameron said that he was "sickened" by the news of the killing.

"[IS] will be defeated, and these sick and barbaric terrorists will face the justice they deserve" he said at a speech in London on Monday.

The latest IS video also shows the beheading of 18 Syrian captives, who are identified as army officers and pilots.

They are said to have been taken from Tabqa air base last August. They are mostly from the Alawite religious sect and come from Latakia and Tartous, according to UK-based monitoring group the Syrian Observatory for Human Rights.

Unlike previous videos released by IS, the latest shows the faces of many of the militants and specifies its location - Dabiq in Syria's Aleppo province.

Image copyright Reuters Image caption Nasser Muthana (centre) appeared in an earlier video, aimed at recruitment

French Interior Minister Bernard Cazeneuve said earlier that Maxime Hauchard, who was born in 1992 and was originally from the Eure region, "had gone to Syria in August 2013 after a stay in Mauritania in 2012".

A third militant was reported to be a Briton, although the father of Nasser Muthana, 20, denied it was his son.

The Daily Mail had earlier reported Ahmed Muthana as saying "it looks like my son".

The man in question stands to the right of another man, who is suspected to be British militant nicknamed "Jihadi John".

He is dressed in black with a balaclava, while the rest of the militants wear army fatigues.

"Jihadi John" has been shown in previous IS videos of the beheadings of the other Western hostages - Britons Alan Henning and David Haines, and US journalists James Foley and Steven Sotloff.

Image copyright Reuters

Abdul-Rahman Kassig

Former US Army Ranger who served in Iraq in 2007

Trained as an emergency medical technician and founded an organisation to supply refugee camps in Syria

Snatched in October 2013 while travelling in eastern Syria for a project

Changed his name from Peter Kassig when he converted to Islam in late 2013

Former soldier and idealist

Kassig's emotional letters home